# SQLObject

Contents:

- SQLObject
  - Credits
  - License
  - Introduction
  - Requirements
  - Compared To Other Database Wrappers
  - Using SQLObject: An Introduction
    - Declaring a Connection
    - Declaring the Class
    - Using the Class
    - Selecting Multiple Objects
      - q-magic
      - selectBy Method
    - Lazy Updates
    - One-to-Many Relationships
    - Many-to-Many Relationships
    - Selecting Objects Using Relationships
    - Class sqlmeta
      - Using sqlmeta
      - j-magic
    - SQLObject Class
    - Customizing the Objects
      - Initializing the Objects
      - Adding Magic Attributes (properties)
      - Overriding Column Attributes
      - Undefined attributes
  - Reference
    - Col Class: Specifying Columns
      - Column Types
    - Relationships Between Classes/Tables
      - ForeignKey
      - MultipleJoin and SQLMultipleJoin: One-to-Many
      - RelatedJoin and SQLRelatedJoin: Many-to-Many
      - SingleJoin: One-to-One
    - Connection pooling
    - Transactions
    - Automatic Schema Generation
      - Indexes
      - Creating and Dropping Tables
  - Dynamic Classes
    - Automatic Class Generation
    - Runtime Column and Join Changes
  - Legacy Database Schemas
    - SQLObject requirements
      - Workaround for primary keys made up of multiple columns
    - Changing the Naming Style
    - Irregular Naming
    - Non-Integer Keys
  - DBConnection: Database Connections
    - MySQL
    - Postgres
    - SQLite
    - Firebird
    - Sybase
    - MAX DB
    - MS SQL Server
  - Events (signals)
  - Exported Symbols
    - LEFT JOIN and other JOINs
    - How can I join a table with itself?
    - Can I use a JOIN() with aliases?
    - Subqueries (subselects)
    - Utilities
    - SQLBuilder

## Credits

SQLObject is by Ian Bicking (ianb@colorstudy.com) and Contributors. The website is sqlobject.org.

## License

The code is licensed under the Lesser General Public License (LGPL).

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

## Introduction

SQLObject is an object-relational mapper for Python programming language. It allows you to translate RDBMS table rows into Python objects, and manipulate those objects to transparently manipulate the database.

In using SQLObject, you will create a class definition that will describe how the object translates to the database table. SQLObject will produce the code to access the database, and update the database with your changes. The generated interface looks similar to any other interface, and callers need not be aware of the database backend.

SQLObject also includes a novel feature to avoid generating, textually, your SQL queries. This also allows non-SQL databases to be used with the same query syntax.

## Requirements

Currently SQLObject supports MySQL and MariaDB via MySQLdb aka MySQL-python (called mysqlclient for Python 3), MySQL Connector, oursql, PyMySQL, mariadb connector, PyODBC and PyPyODBC. For PostgreSQL psycopg2 is recommended; PyGreSQL, py-postgresql and pg8000 are supported; SQLite has a built-in driver, PySQLite or supersqlite. Firebird is supported via fdb or kinterbasdb; pyfirebirdsql is supported but has problems. MAX DB (also known as SAP DB) is supported via sapdb. Sybase via Sybase. MSSQL Server via pymssql (+ FreeTDS) or adodbapi (Win32). PyODBC and PyPyODBC are supported for MySQL, PostgreSQL and MSSQL but have problems (not all tests passed).

Python 2.7 or 3.4+ is required.

## Compared To Other Database Wrappers

There are several object-relational mappers (ORM) for Python. We honestly can’t comment deeply on the quality of those packages, but we’ll try to place SQLObject in perspective.

Objects have built-in magic – setting attributes has side effects (it changes the database), and defining classes has side effects (through the use of metaclasses). Attributes are generally exposed, not marked private, knowing that they can be made dynamic or write-only later.

SQLObject creates objects that feel similar to normal Python objects. An attribute attached to a column doesn’t look different than an attribute that’s attached to a file, or an attribute that is calculated. It is a specific goal that you be able to change the database without changing the interface, including changing the scope of the database, making it more or less prominent as a storage mechanism.

This is in contrast to some ORMs that provide a dictionary-like interface to the database (for example, PyDO). The dictionary interface distinguishes the row from a normal Python object. We also don’t care for the use of strings where an attribute seems more natural – columns are limited in number and predefined, just like attributes. (Note: newer version of PyDO apparently allow attribute access as well)

SQLObject is, to my knowledge, unique in using metaclasses to facilitate this seamless integration. Some other ORMs use code generation to create an interface, expressing the schema in a CSV or XML file (for example, MiddleKit, part of Webware). By using metaclasses you are able to comfortably define your schema in the Python source code. No code generation, no weird tools, no compilation step.

SQLObject provides a strong database abstraction, allowing cross-database compatibility (so long as you don’t sidestep SQLObject).

SQLObject has joins, one-to-many, and many-to-many, something which many ORMs do not have. The join system is also intended to be extensible.

You can map between database names and Python attribute and class names; often these two won’t match, or the database style would be inappropriate for a Python attribute. This way your database schema does not have to be designed with SQLObject in mind, and the resulting classes do not have to inherit the database’s naming schemes.

## Using SQLObject: An Introduction

Let’s start off quickly. We’ll generally just import everything from the `sqlobject` class:

    >>> from sqlobject import *

### Declaring a Connection

The connection URI must follow the standard URI syntax:

    scheme://[user[:password]@]host[:port]/database[?parameters]

Scheme is one of `sqlite`, `mysql`, `postgres`, `firebird`, `interbase`, `maxdb`, `sapdb`, `mssql`, `sybase`.

Examples:

    mysql://user:password@host/database
    mysql://host/database?debug=1
    postgres://user@host/database?debug=&cache=
    postgres:///full/path/to/socket/database
    postgres://host:5432/database
    sqlite:///full/path/to/database
    sqlite:/C:/full/path/to/database
    sqlite:/:memory:

Parameters are: `debug` (default: False), `debugOutput` (default: False), `cache` (default: True), `autoCommit` (default: True), `debugThreading` (default: False), `logger` (default: None), `loglevel` (default: None), `schema` (default: None).

If you want to pass True value in a connection URI - pass almost any non-empty string, especially `yes`, `true`, `on` or `1`; an empty string or `no`, `false`, `off` or `0` for False.

There are also connection-specific parameters, they are listed in the appropriate sections.

Lets first set up a connection:

    >>> import os
    >>> db_filename = os.path.abspath('data.db')
    >>> connection_string = 'sqlite:' + db_filename
    >>> connection = connectionForURI(connection_string)
    >>> sqlhub.processConnection = connection

The `sqlhub.processConnection` assignment means that all classes will, by default, use this connection we’ve just set up.

### Declaring the Class

We’ll develop a simple addressbook-like database. We could create the tables ourselves, and just have SQLObject access those tables, but let’s have SQLObject do that work. First, the class:

    >>> class Person(SQLObject):
    ...
    ...     firstName = StringCol()
    ...     middleInitial = StringCol(length=1, default=None)
    ...     lastName = StringCol()

Many basic table schemas won’t be any more complicated than that. firstName, middleInitial, and lastName are all columns in the database. The general schema implied by this class definition is:

    CREATE TABLE person (
        id INT PRIMARY KEY AUTO_INCREMENT,
        first_name TEXT,
        middle_initial CHAR(1),
        last_name TEXT
    );

This is for SQLite or MySQL. The schema for other databases looks slightly different (especially the `id` column). You’ll notice the names were changed from mixedCase to underscore_separated – this is done by the style object. There are a variety of ways to handle names that don’t fit conventions (see Irregular Naming).

Now we’ll create the table in the database:

    >>> Person.createTable()
    []

We can change the type of the various columns by using something other than StringCol, or using different arguments. More about this in Column Types.

You’ll note that the `id` column is not given in the class definition, it is implied. For MySQL databases it should be defined as `INT PRIMARY KEY AUTO_INCREMENT`, in Postgres `SERIAL PRIMARY KEY`, in SQLite as `INTEGER PRIMARY KEY AUTOINCREMENT`, and for other backends accordingly. You can’t use tables with SQLObject that don’t have a single primary key, and you must treat that key as immutable (otherwise you’ll confuse SQLObject terribly).

You can override the id name in the database, but it is always called `.id` from Python.

### Using the Class

Now that you have a class, how will you use it? We’ll be considering the class defined above.

To create a new object (and row), use class instantiation, like:

    >>> Person(firstName="John", lastName="Doe")
    <Person 1 firstName='John' middleInitial=None lastName='Doe'>

Note

In SQLObject NULL/None does not mean default. NULL is a funny thing; it mean very different things in different contexts and to different people. Sometimes it means “default”, sometimes “not applicable”, sometimes “unknown”. If you want a default, NULL or otherwise, you always have to be explicit in your class definition.

Also note that the SQLObject default isn’t the same as the database’s default (SQLObject never uses the database’s default).

If you had left out `firstName` or `lastName` you would have gotten an error, as no default was given for these columns (`middleInitial` has a default, so it will be set to `NULL`, the database equivalent of `None`).

You can use the class method .get() to fetch instances that already exist:

    >>> Person.get(1)
    <Person 1 firstName='John' middleInitial=None lastName='Doe'>

When you create an object, it is immediately inserted into the database. SQLObject uses the database as immediate storage, unlike some other systems where you explicitly save objects into a database.

Here’s a longer example of using the class:

    >>> p = Person.get(1)
    >>> p
    <Person 1 firstName='John' middleInitial=None lastName='Doe'>
    >>> p.firstName
    'John'
    >>> p.middleInitial = 'Q'
    >>> p.middleInitial
    'Q'
    >>> p2 = Person.get(1)
    >>> p2
    <Person 1 firstName='John' middleInitial='Q' lastName='Doe'>
    >>> p is p2
    True

Columns are accessed like attributes. (This uses the `property` feature of Python, so that retrieving and setting these attributes executes code). Also note that objects are unique – there is generally only one `Person` instance of a particular id in memory at any one time. If you ask for a person by a particular ID more than once, you’ll get back the same instance. This way you can be sure of a certain amount of consistency if you have multiple threads accessing the same data (though of course across processes there can be no sharing of an instance). This isn’t true if you’re using transactions, which are necessarily isolated.

To get an idea of what’s happening behind the surface, we’ll give the same actions with the SQL that is sent, along with some commentary:

    >>> # This will make SQLObject print out the SQL it executes:
    >>> Person._connection.debug = True
    >>> p = Person(firstName='Bob', lastName='Hope')
     1/QueryIns:  INSERT INTO person (first_name, middle_initial, last_name) VALUES ('Bob', NULL, 'Hope')
     1/QueryR  :  INSERT INTO person (first_name, middle_initial, last_name) VALUES ('Bob', NULL, 'Hope')
     1/COMMIT  :  auto
     1/QueryOne:  SELECT first_name, middle_initial, last_name FROM person WHERE ((person.id) = (2))
     1/QueryR  :  SELECT first_name, middle_initial, last_name FROM person WHERE ((person.id) = (2))
     1/COMMIT  :  auto
    >>> p
    <Person 2 firstName='Bob' middleInitial=None lastName='Hope'>
    >>> p.middleInitial = 'Q'
     1/Query   :  UPDATE person SET middle_initial = ('Q') WHERE id = (2)
     1/QueryR  :  UPDATE person SET middle_initial = ('Q') WHERE id = (2)
     1/COMMIT  :  auto
    >>> p2 = Person.get(1)
    >>> # Note: no database access, since we're just grabbing the same
    >>> # instance we already had.

Hopefully you see that the SQL that gets sent is pretty clear and predictable. To view the SQL being sent, add `?debug=true` to your connection URI, or set the `debug` attribute on the connection, and all SQL will be printed to the console. This can be reassuring, and we would encourage you to try it.

As a small optimization, instead of assigning each attribute individually, you can assign a number of them using the `set` method, like:

    >>> p.set(firstName='Robert', lastName='Hope Jr.')

This will send only one `UPDATE` statement. You can also use set with non-database properties (there’s no benefit, but it helps hide the difference between database and non-database attributes).

### Selecting Multiple Objects

While the full power of all the kinds of joins you can do with a relational database are not revealed in SQLObject, a simple `SELECT` is available.

`select` is a class method, and you call it like (with the SQL that’s generated):

    >>> Person._connection.debug = True
    >>> peeps = Person.select(Person.q.firstName=="John")
    >>> list(peeps)
     1/Select  :  SELECT person.id, person.first_name, person.middle_initial, person.last_name FROM person WHERE ((person.first_name) = ('John'))
     1/QueryR  :  SELECT person.id, person.first_name, person.middle_initial, person.last_name FROM person WHERE ((person.first_name) = ('John'))
     1/COMMIT  :  auto
    [<Person 1 firstName='John' middleInitial='Q' lastName='Doe'>]

This example returns everyone with the first name John.

Queries can be more complex:

    >>> peeps = Person.select(
    ...         OR(Person.q.firstName == "John",
    ...            LIKE(Person.q.lastName, "%Hope%")))
    >>> list(peeps)
     1/Select  :  SELECT person.id, person.first_name, person.middle_initial, person.last_name FROM person WHERE (((person.first_name) = ('John')) OR (person.last_name LIKE ('%Hope%')))
     1/QueryR  :  SELECT person.id, person.first_name, person.middle_initial, person.last_name FROM person WHERE (((person.first_name) = ('John')) OR (person.last_name LIKE ('%Hope%')))
     1/COMMIT  :  auto
    [<Person 1 firstName='John' middleInitial='Q' lastName='Doe'>, <Person 2 firstName='Robert' middleInitial='Q' lastName='Hope Jr.'>]

You’ll note that classes have an attribute `q`, which gives access to special objects for constructing query clauses. All attributes under `q` refer to column names and if you construct logical statements with these it’ll give you the SQL for that statement. You can also create your SQL more manually:

    >>> Person._connection.debug = False  # Need for doctests
    >>> peeps = Person.select("""person.first_name = 'John' AND
    ...                          person.last_name LIKE 'D%'""")

You should use MyClass.sqlrepr to quote any values you use if you create SQL manually (quoting is automatic if you use `q`).

You can use the keyword arguments orderBy to create `ORDER BY` in the select statements: orderBy takes a string, which should be the database name of the column, or a column in the form `Person.q.firstName`. You can use `"-colname"` or `DESC(Person.q.firstName`) to specify descending order (this is translated to DESC, so it works on non-numeric types as well), or call `MyClass.select().reversed()`. orderBy can also take a list of columns in the same format: `["-weight", "name"]`.

You can use the sqlmeta class variable defaultOrder to give a default ordering for all selects. To get an unordered result when defaultOrder is used, use `orderBy=None`.

Select results are generators, which are lazily evaluated. So the SQL is only executed when you iterate over the select results, or if you use `list()` to force the result to be executed. When you iterate over the select results, rows are fetched one at a time. This way you can iterate over large results without keeping the entire result set in memory. You can also do things like `.reversed()` without fetching and reversing the entire result – instead, SQLObject can change the SQL that is sent so you get equivalent results.

You can also slice select results. This modifies the SQL query, so `peeps[:10]` will result in `LIMIT 10` being added to the end of the SQL query. If the slice cannot be performed in the SQL (e.g., peeps[:-10]), then the select is executed, and the slice is performed on the list of results. This will generally only happen when you use negative indexes.

In certain cases, you may get a select result with an object in it more than once, e.g., in some joins. If you don’t want this, you can add the keyword argument `MyClass.select(..., distinct=True)`, which results in a `SELECT DISTINCT` call.

You can get the length of the result without fetching all the results by calling `count` on the result object, like `MyClass.select().count()`. A `COUNT(*)` query is used – the actual objects are not fetched from the database. Together with slicing, this makes batched queries easy to write:

> start = 20 size = 10 query = Table.select() results = query[start:start+size] total = query.count() print “Showing page %i of %i” % (start/size + 1, total/size + 1)

Note

There are several factors when considering the efficiency of this kind of batching, and it depends very much how the batching is being used. Consider a web application where you are showing an average of 100 results, 10 at a time, and the results are ordered by the date they were added to the database. While slicing will keep the database from returning all the results (and so save some communication time), the database will still have to scan through the entire result set to sort the items (so it knows which the first ten are), and depending on your query may need to scan through the entire table (depending on your use of indexes). Indexes are probably the most important way to improve importance in a case like this, and you may find caching to be more effective than slicing.

In this case, caching would mean retrieving the complete results. You can use `list(MyClass.select(...))` to do this. You can save these results for some limited period of time, as the user looks through the results page by page. This means the first page in a search result will be slightly more expensive, but all later pages will be very cheap.

For more information on the where clause in the queries, see the SQLBuilder documentation.

#### q-magic

Please note the use of the q attribute in examples above. q is an object that returns special objects to construct SQL expressions. Operations on objects returned by q-magic are not evaluated immediately but stored in a manner similar to symbolic algebra; the entire expression is evaluated by constructing a string that is sent then to the backend.

For example, for the code:

    >>> peeps = Person.select(Person.q.firstName=="John")

SQLObject doesn’t evaluate firstName but stores the expression:

> Person.q.firstName==”John”

Later SQLObject converts it to the string `first_name = 'John'` and passes the string to the backend.

#### selectBy Method

An alternative to `.select` is `.selectBy`. It works like:

    >>> peeps = Person.selectBy(firstName="John", lastName="Doe")

Each keyword argument is a column, and all the keyword arguments are ANDed together. The return value is a SelectResults, so you can slice it, count it, order it, etc.

### Lazy Updates

By default SQLObject sends an `UPDATE` to the database for every attribute you set, or every time you call `.set()`. If you want to avoid this many updates, add `lazyUpdate = True` to your class sqlmeta definition.

Then updates will only be written to the database when you call `inst.syncUpdate()` or `inst.sync()`: `.sync()` also refetches the data from the database, which `.syncUpdate()` does not do.

When enabled instances will have a property `.sqlmeta.dirty`, which indicates if there are pending updates. Inserts are still done immediately; there’s no way to do lazy inserts at this time.

### One-to-Many Relationships

An address book is nothing without addresses.

First, let’s define the new address table. People can have multiple addresses, of course:

    >>> class Address(SQLObject):
    ...
    ...     street = StringCol()
    ...     city = StringCol()
    ...     state = StringCol(length=2)
    ...     zip = StringCol(length=9)
    ...     person = ForeignKey('Person')
    >>> Address.createTable()
    []

Note the column `person = ForeignKey("Person")`. This is a reference to a Person object. We refer to other classes by name (with a string). In the database there will be a `person_id` column, type `INT`, which points to the `person` column.

Note

The reason SQLObject uses strings to refer to other classes is because the other class often does not yet exist. Classes in Python are created, not declared; so when a module is imported the commands are executed. `class` is just another command; one that creates a class and assigns it to the name you give.

If class `A` referred to class `B`, but class `B` was defined below `A` in the module, then when the `A` class was created (including creating all its column attributes) the `B` class simply wouldn’t exist. By referring to classes by name, we can wait until all the required classes exist before creating the links between classes.

We want an attribute that gives the addresses for a person. In a class definition we’d do:

    class Person(SQLObject):
        ...
        addresses = MultipleJoin('Address')

But we already have the class. We can add this to the class in-place:

    >>> Person.sqlmeta.addJoin(MultipleJoin('Address',
    ...                        joinMethodName='addresses'))

Note

In almost all cases you can modify SQLObject classes after they’ve been created. Having attributes that contain `*Col` objects in the class definition is equivalent to calling certain class methods (like `addColumn()`).

Now we can get the backreference with `aPerson.addresses`, which returns a list. An example:

    >>> p.addresses
    []
    >>> Address(street='123 W Main St', city='Smallsville',
    ...         state='MN', zip='55407', person=p)
    <Address 1 ...>
    >>> p.addresses
    [<Address 1 ...>]

Note

MultipleJoin, as well as RelatedJoin, returns a list of results. It is often preferable to get a SelectResults object instead, in which case you should use SQLMultipleJoin and SQLRelatedJoin. The declaration of these joins is unchanged from above, but the returned iterator has many additional useful methods.

### Many-to-Many Relationships

For this example we will have user and role objects. The two have a many-to-many relationship, which is represented with the RelatedJoin.

    >>> class User(SQLObject):
    ...
    ...     class sqlmeta:
    ...         # user is a reserved word in some databases, so we won't
    ...         # use that for the table name:
    ...         table = "user_table"
    ...
    ...     username = StringCol(alternateID=True, length=20)
    ...     # We'd probably define more attributes, but we'll leave
    ...     # that exercise to the reader...
    ...
    ...     roles = RelatedJoin('Role')

    >>> class Role(SQLObject):
    ...
    ...     name = StringCol(alternateID=True, length=20)
    ...
    ...     users = RelatedJoin('User')

    >>> User.createTable()
    []
    >>> Role.createTable()
    []

Note

The sqlmeta class is used to store different kinds of metadata (and override that metadata, like table). This is new in SQLObject 0.7. See the section Class sqlmeta for more information on how it works and what attributes have special meanings.

And usage:

    >>> bob = User(username='bob')
    >>> tim = User(username='tim')
    >>> jay = User(username='jay')
    >>> admin = Role(name='admin')
    >>> editor = Role(name='editor')
    >>> bob.addRole(admin)
    >>> bob.addRole(editor)
    >>> tim.addRole(editor)
    >>> bob.roles
    [<Role 1 name='admin'>, <Role 2 name='editor'>]
    >>> tim.roles
    [<Role 2 name='editor'>]
    >>> jay.roles
    []
    >>> admin.users
    [<User 1 username='bob'>]
    >>> editor.users
    [<User 1 username='bob'>, <User 2 username='tim'>]

In the process an intermediate table is created, `role_user`, which references both of the other classes. This table is never exposed as a class, and its rows do not have equivalent Python objects – this hides some of the nuisance of a many-to-many relationship.

By the way, if you want to create an intermediate table of your own, maybe with additional columns, be aware that the standard SQLObject methods add/removesomething may not work as expected. Assuming that you are providing the join with the correct joinColumn and otherColumn arguments, be aware it’s not possible to insert extra data via such methods, nor will they set any default value.

Let’s have an example: in the previous User/Role system, you’re creating a UserRole intermediate table, with the two columns containing the foreign keys for the MTM relationship, and an additional DateTimeCol defaulting to datetime.datetime.now : that column will stay empty when adding roles with the addRole method. If you want to get a list of rows from the intermediate table directly add a MultipleJoin to User or Role class.

You may notice that the columns have the extra keyword argument alternateID. If you use `alternateID=True`, this means that the column uniquely identifies rows – like a username uniquely identifies a user. This identifier is in addition to the primary key (`id`), which is always present.

Note

SQLObject has a strong requirement that the primary key be unique and immutable. You cannot change the primary key through SQLObject, and if you change it through another mechanism you can cause inconsistency in any running SQLObject program (and in your data). For this reason meaningless integer IDs are encouraged – something like a username that could change in the future may uniquely identify a row, but it may be changed in the future. So long as it is not used to reference the row, it is also safe to change it in the future.

A alternateID column creates a class method, like `byUsername` for a column named `username` (or you can use the alternateMethodName keyword argument to override this). Its use:

    >>> User.byUsername('bob')
    <User 1 username='bob'>
    >>> Role.byName('admin')
    <Role 1 name='admin'>

### Selecting Objects Using Relationships

An select expression can refer to multiple classes, like:

    >>> Person._connection.debug = False # Needed for doctests
    >>> peeps = Person.select(
    ...         AND(Address.q.personID == Person.q.id,
    ...             Address.q.zip.startswith('504')))
    >>> list(peeps)
    []
    >>> peeps = Person.select(
    ...         AND(Address.q.personID == Person.q.id,
    ...             Address.q.zip.startswith('554')))
    >>> list(peeps)
    [<Person 2 firstName='Robert' middleInitial='Q' lastName='Hope Jr.'>]

It is also possible to use the `q` attribute when constructing complex queries, like:

    >>> Person._connection.debug = False  # Needed for doctests
    >>> peeps = Person.select("""address.person_id = person.id AND
    ...                          address.zip LIKE '504%'""",
    ...                       clauseTables=['address'])

Note that you have to use `clauseTables` if you use tables besides the one you are selecting from. If you use the `q` attributes SQLObject will automatically figure out what extra classes you might have used.

### Class sqlmeta

This new class is available starting with SQLObject 0.7 and allows specifying metadata in a clearer way, without polluting the class namespace with more attributes.

There are some special attributes that can be used inside this class that will change the behavior of the class that contains it. Those values are:

table:
  The name of the table in the database. This is derived from `style` and the class name if no explicit name is given. If you don’t give a name and haven’t defined an alternative `style`, then the standard MixedCase to mixed_case translation is performed.
idName:
  The name of the primary key column in the database. This is derived from `style` if no explicit name is given. The default name is `id`.
idType:
  A function that coerces/normalizes IDs when setting IDs. This is `int` by default (all IDs are normalized to integers).
style:
  A style object – this object allows you to use other algorithms for translating between Python attribute and class names, and the database’s column and table names. See Changing the Naming Style for more. It is an instance of the IStyle interface.
lazyUpdate:
  A boolean (default false). If true, then setting attributes on instances (or using `inst.set(.)` will not send `UPDATE` queries immediately (you must call `inst.syncUpdates()` or `inst.sync()` first).
defaultOrder:
  When selecting objects and not giving an explicit order, this attribute indicates the default ordering. It is like this value is passed to `.select()` and related methods; see those method’s documentation for details.
cacheValues:

  A boolean (default true). If true, then the values in the row are cached as long as the instance is kept (and `inst.expire()` is not called).

  If set to False then values for attributes from the database won’t be cached. So every time you access an attribute in the object the database will be queried for a value, i.e., a `SELECT` will be issued. If you want to handle concurrent access to the database from multiple processes then this is probably the way to do so.

registry:
  Because SQLObject uses strings to relate classes, and these strings do not respect module names, name clashes will occur if you put different systems together. This string value serves as a namespace for classes.
fromDatabase:
  A boolean (default false). If true, then on class creation the database will be queried for the table’s columns, and any missing columns (possible all columns) will be added automatically. Please be warned that not all connections fully implement database introspection.
dbEncoding:
  UnicodeCol looks up sqlmeta.dbEncoding if column.dbEncoding is `None` (if sqlmeta.dbEncoding is `None` UnicodeCol looks up connection.dbEncoding and if dbEncoding isn’t defined anywhere it defaults to `"utf-8"`). For Python 3 there must be one encoding for connection - do not define different columns with different encodings, it’s not implemented.

The following attributes provide introspection but should not be set directly - see Runtime Column and Join Changes for dynamically modifying these class elements.

columns:
  A dictionary of `{columnName: anSOColInstance}`. You can get information on the columns via this read-only attribute.
columnList:
  A list of the values in `columns`. Sometimes a stable, ordered version of the columns is necessary; this is used for that.
columnDefinitions:
  A dictionary like `columns`, but contains the original column definitions (which are not class-specific, and have no logic).
joins:
  A list of all the Join objects for this class.
indexes:
  A list of all the indexes for this class.
createSQL:
  SQL queries run after table creation. createSQL can be a string with a single SQL command, a list of SQL commands, or a dictionary with keys that are dbNames and values that are either single SQL command string or a list of SQL commands. This is usually for ALTER TABLE commands.

There is also one instance attribute:

expired:
  A boolean. If true, then the next time this object’s column attributes are accessed a query will be run.

While in previous versions of SQLObject those attributes were defined directly at the class that will map your database data to Python and all of them were prefixed with an underscore, now it is suggested that you change your code to this new style. The old way was removed in SQLObject 0.8.

Please note: when using InheritedSQLObject, sqlmeta attributes don’t get inherited, e.g. you can’t access via the sqlmeta.columns dictionary the parent’s class column objects.

#### Using sqlmeta

To use sqlmeta you should write code like this example:

    class MyClass(SQLObject):

        class sqlmeta:
            lazyUpdate = True
            cacheValues = False

        columnA = StringCol()
        columnB = IntCol()

        def _set_attr1(self, value):
            # do something with value

        def _get_attr1(self):
            # do something to retrieve value

The above definition is creating a table `my_class` (the name may be different if you change the `style` used) with two columns called columnA and columnB. There’s also a third field that can be accessed using `MyClass.attr1`. The sqlmeta class is changing the behavior of `MyClass` so that it will perform lazy updates (you’ll have to call the `.sync()` method to write the updates to the database) and it is also telling that `MyClass` won’t have any cache, so that every time you ask for some information it will be retrieved from the database.

#### j-magic

There is a magic attribute j similar to q with attributes for ForeignKey and SQLMultipleJoin/SQLRelatedJoin, providing a shorthand for the SQLBuilder join expressions to traverse the given relationship. For example, for a ForeignKey AClass.j.someB is equivalent to (AClass.q.someBID==BClass.q.id), as is BClass.j.someAs for the matching SQLMultipleJoin.

### SQLObject Class

There is one special attribute - _connection. It is the connection defined for the table.

_connection:

  The connection object to use, from DBConnection. You can also set the variable __connection__ in the enclosing module and it will be picked up (be sure to define __connection__ before your class). You can also pass a connection object in at instance creation time, as described in transactions.

  If you have defined sqlhub.processConnection then this attribute can be omitted from your class and the sqlhub will be used instead. If you have several classes using the same connection that might be an advantage, besides saving a lot of typing.

### Customizing the Objects

While we haven’t done so in the examples, you can include your own methods in the class definition. Writing your own methods should be obvious enough (just do so like in any other class), but there are some other details to be aware of.

#### Initializing the Objects

There are two ways SQLObject instances can come into existence: they can be fetched from the database, or they can be inserted into the database. In both cases a new Python object is created. This makes the role of __init__ a little confusing.

In general, you should not touch __init__. Instead use the _init method, which is called after an object is fetched or inserted. This method has the signature `_init(self, id, connection=None, selectResults=None)`, though you may just want to use `_init(self, *args, **kw)`. Note: don’t forget to call `SQLObject._init(self, *args, **kw)` if you override the method!

#### Adding Magic Attributes (properties)

You can use all the normal techniques for defining methods in this class, including classmethod, staticmethod, and property, but you can also use a shortcut. If you have a method that’s name starts with `_set_`, `_get_`, `_del_`, or `_doc_`, it will be used to create a property. So, for instance, say you have images stored under the ID of the person in the `/var/people/images` directory:

    class Person(SQLObject):
        # ...

        def imageFilename(self):
            return 'images/person-%s.jpg' % self.id

        def _get_image(self):
            if not os.path.exists(self.imageFilename()):
                return None
            f = open(self.imageFilename())
            v = f.read()
            f.close()
            return v

        def _set_image(self, value):
            # assume we get a string for the image
            f = open(self.imageFilename(), 'w')
            f.write(value)
            f.close()

        def _del_image(self, value):
            # We usually wouldn't include a method like this, but for
            # instructional purposes...
            os.unlink(self.imageFilename())

Later, you can use the `.image` property just like an attribute, and the changes will be reflected in the filesystem by calling these methods. This is a good technique for information that is better to keep in files as opposed to the database (such as large, opaque data like images).

You can also pass an `image` keyword argument to the constructor or the set method, like `Person(..., image=imageText)`.

All of the methods (`_get_`, `_set_`, etc) are optional – you can use any one of them without using the others. So you could define just a `_get_attr` method so that `attr` was read-only.

#### Overriding Column Attributes

It’s a little more complicated if you want to override the behavior of an database column attribute. For instance, imagine there’s special code you want to run whenever someone’s name changes. In many systems you’d do some custom code, then call the superclass’s code. But the superclass (`SQLObject`) doesn’t know anything about the column in your subclass. It’s even worse with properties.

SQLObject creates methods like `_set_lastName` for each of your columns, but again you can’t use this, since there’s no superclass to reference (and you can’t write `SQLObject._set_lastName(...)`, because the SQLObject class doesn’t know about your class’s columns). You want to override that `_set_lastName` method yourself.

To deal with this, SQLObject creates two methods for each getter and setter, for example: `_set_lastName` and `_SO_set_lastName`. So to intercept all changes to `lastName`:

    class Person(SQLObject):
        lastName = StringCol()
        firstName = StringCol()

        def _set_lastName(self, value):
            self.notifyLastNameChange(value)
            self._SO_set_lastName(value)

Or perhaps you want to constrain a phone numbers to be actual digits, and of proper length, and make the formatting nice:

    import re

    class PhoneNumber(SQLObject):
        phoneNumber = StringCol(length=30)

        _garbageCharactersRE = re.compile(r'[\-\.\(\) ]')
        _phoneNumberRE = re.compile(r'^[0-9]+$')
        def _set_phoneNumber(self, value):
            value = self._garbageCharactersRE.sub('', value)
            if not len(value) >= 10:
                raise ValueError(
                    'Phone numbers must be at least 10 digits long')
            if not self._phoneNumberRE.match(value):
                raise ValueError, 'Phone numbers can contain only digits'
            self._SO_set_phoneNumber(value)

        def _get_phoneNumber(self):
            value = self._SO_get_phoneNumber()
            number = '(%s) %s-%s' % (value[0:3], value[3:6], value[6:10])
            if len(value) > 10:
                number += ' ext.%s' % value[10:]
            return number

Note

You should be a little cautious when modifying data that gets set in an attribute. Generally someone using your class will expect that the value they set the attribute to will be the same value they get back. In this example we removed some of the characters before putting it in the database, and reformatted it on the way out. One advantage of methods (as opposed to attribute access) is that the programmer is more likely to expect this disconnect.

Also note while these conversions will take place when getting and setting the column, in queries the conversions will not take place. So if you convert the value from a “Pythonic” representation to a “SQLish” representation, your queries (when using `.select()` and `.selectBy()`) will have to be in terms of the SQL/Database representation (as those commands generate SQL that is run on the database).

#### Undefined attributes

There’s one more thing worth telling, because you may something get strange results when making a typo. SQLObject won’t ever complain or raise any error when setting a previously undefined attribute; it will simply set it, without making any change to the database, i.e: it will work as any other attribute you set on any Python class, it will ‘forget’ it is a SQLObject class.

This may sometimes be a problem: if you have got a ‘name’ attribute and you you write `a.namme="Victor"` once, when setting it, you’ll get no error, no warning, nothing at all, and you may get crazy at understanding why you don’t get that value set in your DB.

## Reference

The instructions above should tell you enough to get you started, and be useful for many situations. Now we’ll show how to specify the class more completely.

### Col Class: Specifying Columns

The list of columns is a list of Col objects. These objects don’t have functionality in themselves, but give you a way to specify the column.

dbName:
  This is the name of the column in the database. If you don’t give a name, your Pythonic name will be converted from mixed-case to underscore-separated.
default:
  The default value for this column. Used when creating a new row. If you give a callable object or function, the function will be called, and the return value will be used. So you can give `DateTimeCol.now` to make the default value be the current time. Or you can use `sqlbuilder.func.NOW()` to have the database use the `NOW()` function internally. If you don’t give a default there will be an exception if this column isn’t specified in the call to new.
defaultSQL:
  `DEFAULT` SQL attribute.
alternateID:

  This boolean (default False) indicates if the column can be used as an ID for the field (for instance, a username), though it is not a primary key. If so a class method will be added, like `byUsername` which will return that object. Use alternateMethodName if you don’t like the `by*` name (e.g. `alternateMethodName="username"`).

  The column should be declared `UNIQUE` in your table schema.

unique:
  If true, when SQLObject creates a table it will declare this column to be `UNIQUE`.
notNone:
  If true, None/`NULL` is not allowed for this column. Useful if you are using SQLObject to create your tables.
sqlType:
  The SQL type for this column (like `INT`, `BOOLEAN`, etc). You can use classes (defined below) for this, but if those don’t work it’s sometimes easiest just to use sqlType. Only necessary if SQLObject is creating your tables.
validator:
  formencode-like validator. Making long story short, this is an object that provides `to_python()` and `from_python()` to validate and convert (adapt or cast) the values when they are read/written from/to the database. You should see formencode validator documentation for more details. This validator is appended to the end of the list of the list of column validators. If the column has a list of validators their `from_python()` methods are ran from the beginnig of the list to the end; `to_python()` in the reverse order. That said, `from_python()` method of this validator is called last, after all validators in the list; `to_python()` is called first.
validator2:
  Another validator. It is inserted in the beginning of the list of the list of validators, i.e. its `from_python()` method is called first; `to_python()` last.

#### Column Types

The ForeignKey class should be used instead of Col when the column is a reference to another table/class. It is generally used like `ForeignKey('Role')`, in this instance to create a reference to a table Role. This is largely equivalent to `Col(foreignKey='Role', sqlType='INT')`. Two attributes will generally be created, `role`, which returns a Role instance, and `roleID`, which returns an integer ID for the related role.

There are some other subclasses of Col. These are used to indicate different types of columns, when SQLObject creates your tables.

BLOBCol:
  A column for binary data. Presently works only with MySQL, PostgreSQL and SQLite backends.
BoolCol:
  Will create a `BOOLEAN` column in Postgres, or `INT` in other databases. It will also convert values to `"t"/"f"` or `0/1` according to the database backend.
CurrencyCol:
  Equivalent to `DecimalCol(size=10, precision=2)`. WARNING: as DecimalCol MAY NOT return precise numbers, this column may share the same behavior. Please read the DecimalCol warning.
DateTimeCol:
  A date and time (usually returned as an datetime or mxDateTime object).
DateCol:
  A date (usually returned as an datetime or mxDateTime object).
TimeCol:
  A time (usually returned as an datetime or mxDateTime object).
TimestampCol:
  Supports MySQL TIMESTAMP type.
DecimalCol:
  Base-10, precise number. Uses the keyword arguments size for number of digits stored, and precision for the number of digits after the decimal point. WARNING: it may happen that DecimalCol values, although correctly stored in the DB, are returned as floats instead of decimals. For example, due to the type affinity SQLite stores decimals as integers or floats (NUMERIC storage class). You should test with your database adapter, and you should try importing the Decimal type and your DB adapter before importing SQLObject.

DecimalStringCol:
  Similar to DecimalCol but stores data as strings to work around problems in some drivers and type affinity problem in SQLite. As it stores data as strings the column cannot be used in SQL expressions (column1 + column2) and probably will has problems with ORDER BY.
EnumCol:

  One of several string values – give the possible strings as a list, with the enumValues keyword argument. MySQL has a native `ENUM` type, but will work with other databases too (storage just won’t be as efficient).

  For PostgreSQL, EnumCol’s are implemented using check constraints. Due to the way PostgreSQL handles check constraints involving NULL, specifying None as a member of an EnumCol will effectively mean that, at the SQL level, the check constraint will be ignored (see http://archives.postgresql.org/pgsql-sql/2004-12/msg00065.php for more details).

SetCol:
  Supports MySQL SET type.
FloatCol:
  Floats.
ForeignKey:
  A key to another table/class. Use like `user = ForeignKey('User')`. It can check for referential integrity using the keyword argument cascade, please see ForeignKey for details.
IntCol:
  Integers.
JsonbCol:
  A column for jsonb objects. Only supported on Postgres. Any Python object that can be serialized with json.dumps can be stored.
JSONCol:
  A universal json column that converts simple Python objects (None, bool, int, float, long, dict, list, str/unicode to/from JSON using json.dumps/loads. A subclass of StringCol. Requires `VARCHAR`/`TEXT` columns at backends, doesn’t work with `JSON` columns.
PickleCol:
  An extension of BLOBCol; this column can store/retrieve any Python object; it actually (un)pickles the object from/to string and stores/retrieves the string. One can get and set the value of the column but cannot search (use it in WHERE).
StringCol:

  A string (character) column. Extra keywords:

  length:
    If given, the type will be something like `VARCHAR(length)`. If not given, then `TEXT` is assumed (i.e., lengthless).
  varchar:
    A boolean; if you have a length, differentiates between `CHAR` and `VARCHAR`, default True, i.e., use `VARCHAR`.
UnicodeCol:

  A subclass of StringCol. Also accepts a dbEncoding keyword argument, it defaults to `None` which means to lookup dbEncoding in sqlmeta and connection, and if dbEncoding isn’t defined anywhere it defaults to `"utf-8"`. Values coming in and out from the database will be encoded and decoded. Note: there are some limitations on using UnicodeCol in queries:

  - only simple q-magic fields are supported; no expressions;
  - only == and != operators are supported;

  The following code works:

      MyTable.select(u'value' == MyTable.q.name)
      MyTable.select(MyTable.q.name != u'value')
      MyTable.select(OR(MyTable.q.col1 == u'value1', MyTable.q.col2 != u'value2'))
      MyTable.selectBy(name = u'value')
      MyTable.selectBy(col1=u'value1', col2=u'value2')
      MyTable.byCol1(u'value1') # if col1 is an alternateID

  The following does not work:

      MyTable.select((MyTable.q.name + MyTable.q.surname) == u'value')

  In that case you must apply the encoding yourself:

      MyTable.select((MyTable.q.name + MyTable.q.surname) == u'value'.encode(dbEncoding))

UuidCol:
  A column for UUID. On Postgres uses ‘UUID’ data type, on all other backends uses VARCHAR(36).

### Relationships Between Classes/Tables

#### ForeignKey

You can use the ForeignKey to handle foreign references in a table, but for back references and many-to-many relationships you’ll use joins.

ForeignKey allows you to specify referential integrity using the keyword cascade, which can have these values:

None:
  No action is taken on related deleted columns (this is the default). Following the Person/Address example, if you delete the object Person with id 1 (John Doe), the Address with id 1 (123 W Main St) will be kept untouched (with `personID=1`).
False:
  Deletion of an object that has other objects related to it using a ForeignKey will fail (sets `ON DELETE RESTRICT`). Following the Person/Address example, if you delete the object Person with id 1 (John Doe) a SQLObjectIntegrityError exception will be raised, because the Address with id 1 (123 W Main St) has a reference (`personID=1`) to it.
True:
  Deletion of an object that has other objects related to it using a ForeignKey will delete all the related objects too (sets `ON DELETE CASCADE`). Following the Person/Address example, if you delete the object Person with id 1 (John Doe), the Address with id 1 (123 W Main St) will be deleted too.
‘null’:
  Deletion of an object that has other objects related to it using a ForeignKey will set the ForeignKey column to NULL/None (sets `ON DELETE SET NULL`). Following the Person/Address example, if you delete the object Person with id 1 (John Doe), the Address with id 1 (123 W Main St) will be kept but the reference to person will be set to NULL/None (`personID=None`).

#### MultipleJoin and SQLMultipleJoin: One-to-Many

See One-to-Many Relationships for an example of one-to-many relationships.

MultipleJoin returns a list of results, while SQLMultipleJoin returns a SelectResults object.

Several keyword arguments are allowed to the MultipleJoin constructor:

joinColumn:
  The column name of the key that points to this table. So, if you have a table `Product`, and another table has a column `ProductNo` that points to this table, then you’d use `joinColumn="ProductNo"`. WARNING: the argument you pass must conform to the column name in the database, not to the column in the class. So, if you have a SQLObject containing the `ProductNo` column, this will probably be translated into `product_no_id` in the DB (`product_no` is the normal uppercase- to-lowercase + underscores SQLO Translation, the added _id is just because the column referring to the table is probably a ForeignKey, and SQLO translates foreign keys that way). You should pass that parameter.
orderBy:
  Like the orderBy argument to select(), you can specify the order that the joined objects should be returned in. defaultOrder will be used if not specified; `None` forces unordered results.
joinMethodName:
  When adding joins dynamically (using the class method addJoin), you can give the name of the accessor for the join. It can also be created automatically, and is normally implied (i.e., `addresses = MultipleJoin(...)` implies `joinMethodName="addresses"`).

#### RelatedJoin and SQLRelatedJoin: Many-to-Many

See Many-to-Many Relationships for examples of using many-to-many joins.

RelatedJoin returns a list of results, while SQLRelatedJoin returns a SelectResults object.

RelatedJoin has all the keyword arguments of MultipleJoin, plus:

otherColumn:
  Similar to joinColumn, but referring to the joined class. Same warning about column name.
intermediateTable:
  The name of the intermediate table which references both classes. WARNING: you should pass the database table name, not the SQLO class representing.
addRemoveName:
  In the user/role example, the methods addRole(role) and removeRole(role) are created. The `Role` portion of these method names can be changed by giving a string value here.
createRelatedTable:
  default: `True`. If `False`, then the related table won’t be automatically created; instead you must manually create it (e.g., with explicit SQLObject classes for the joins). New in 0.7.1.

Note

Let’s suppose you have SQLObject-inherited classes Alpha and Beta, and an AlphasAndBetas used for the many-to-many relationship. AlphasAndBetas contains the alphaIndex Foreign Key column referring to Alpha, and the betaIndex FK column referring to Beta. if you want a ‘betas’ RelatedJoin in Alpha, you should add it to Alpha passing ‘Beta’ (class name!) as the first parameter, then passing ‘alpha_index_id’ as joinColumn, ‘beta_index_id’ as otherColumn, and ‘alphas_and_betas’ as intermediateTable.

An example schema that requires the use of joinColumn, otherColumn, and intermediateTable:

    CREATE TABLE person (
        id SERIAL,
        username VARCHAR(100) NOT NULL UNIQUE
    );

    CREATE TABLE role (
        id SERIAL,
        name VARCHAR(50) NOT NULL UNIQUE
    );

    CREATE TABLE assigned_roles (
        person INT NOT NULL,
        role INT NOT NULL
    );

Then the usage in a class:

    class Person(SQLObject):
        username = StringCol(length=100, alternateID=True)
        roles = RelatedJoin('Role', joinColumn='person', otherColumn='role',
                            intermediateTable='assigned_roles')
    class Role(SQLObject):
        name = StringCol(length=50, alternateID=True)
        roles = RelatedJoin('Person', joinColumn='role', otherColumn='person',
                            intermediateTable='assigned_roles')

#### SingleJoin: One-to-One

Similar to MultipleJoin, but returns just one object, not a list.

### Connection pooling

Connection object acquires a new low-level DB API connection from the pool and stores it; the low-level connection is removed from the pool; “releasing” means “return it to the pool”. For single-threaded programs there is one connection in the pool.

If the pool is empty a new low-level connection opened; if one has disabled pooling (by setting conn._pool = None) the connection will be closed instead of returning to the pool.

### Transactions

Transaction support in SQLObject is left to the database. Transactions can be used like:

    conn = DBConnection.PostgresConnection('yada')
    trans = conn.transaction()
    p = Person.get(1, trans)
    p.firstName = 'Bob'
    trans.commit()
    p.firstName = 'Billy'
    trans.rollback()

The `trans` object here is essentially a wrapper around a single database connection, and commit and rollback just pass that message to the low-level connection.

One can call as much `.commit()`’s, but after a `.rollback()` one has to call `.begin()`. The last `.commit()` should be called as `.commit(close=True)` to release low-level connection back to the connection pool.

You can use SELECT FOR UPDATE in those databases that support it:

    Person.select(Person.q.name=="value", forUpdate=True, connection=trans)

Method `sqlhub.doInTransaction` can be used to run a piece of code in a transaction. The method accepts a callable and positional and keywords arguments. It begins a transaction using its `processConnection` or `threadConnection`, calls the callable, commits the transaction and closes the underlying connection; it returns whatever the callable returned. If an error occurs during call to the callable it rolls the transaction back and reraise the exception.

### Automatic Schema Generation

All the connections support creating and dropping tables based on the class definition. First you have to prepare your class definition, which means including type information in your columns.

#### Indexes

You can also define indexes for your tables, which is only meaningful when creating your tables through SQLObject (SQLObject relies on the database to implement the indexes). You do this again with attribute assignment, like:

    firstLastIndex = DatabaseIndex('firstName', 'lastName')

This creates an index on two columns, useful if you are selecting a particular name. Of course, you can give a single column, and you can give the column object (`firstName`) instead of the string name. Note that if you use `unique` or `alternateID` (which implies `unique`) the database may make an index for you, and primary keys are always indexed.

If you give the keyword argument `unique` to DatabaseIndex you’ll create a unique index – the combination of columns must be unique.

You can also use dictionaries in place of the column names, to add extra options. E.g.:

    lastNameIndex = DatabaseIndex({'expression': 'lower(last_name)'})

In that case, the index will be on the lower-case version of the column. It seems that only PostgreSQL supports this. You can also do:

    lastNameIndex = DatabaseIndex({'column': lastName, 'length': 10})

Which asks the database to only pay attention to the first ten characters. Only MySQL supports this, but it is ignored in other databases.

#### Creating and Dropping Tables

To create a table call createTable. It takes two arguments:

ifNotExists:
  If the table already exists, then don’t try to create it. Default False.
createJoinTables:
  If you used Many-to-Many relationships, then the intermediate tables will be created (but only for one of the two involved classes). Default True.

dropTable takes arguments ifExists and dropJoinTables, self-explanatory.

## Dynamic Classes

SQLObject classes can be manipulated dynamically. This leaves open the possibility of constructing SQLObject classes from an XML file, from database introspection, or from a graphical interface.

### Automatic Class Generation

SQLObject can read the table description from the database, and fill in the class columns (as would normally be described in the _columns attribute). Do this like:

    class Person(SQLObject):
        class sqlmeta:
            fromDatabase = True

You can still specify columns (in _columns), and only missing columns will be added.

### Runtime Column and Join Changes

You can add and remove columns to your class at runtime. Such changes will effect all instances, since changes are made in place to the class. There are two methods of the class sqlmeta object, addColumn and delColumn, both of which take a Col object (or subclass) as an argument. There’s also an option argument changeSchema which, if True, will add or drop the column from the database (typically with an `ALTER` command).

When adding columns, you must pass the name as part of the column constructor, like `StringCol("username", length=20)`. When removing columns, you can either use the Col object (as found in sqlmeta.columns, or which you used in addColumn), or you can use the column name (like `MyClass.delColumn("username")`).

You can also add Joins, like `MyClass.addJoin(MultipleJoin("MyOtherClass"))`, and remove joins with delJoin. delJoin does not take strings, you have to get the join object out of the sqlmeta.joins attribute.

## Legacy Database Schemas

Often you will have a database that already exists, and does not use the naming conventions that SQLObject expects, or does not use any naming convention at all.

### SQLObject requirements

While SQLObject tries not to make too many requirements on your schema, some assumptions are made. Some of these may be relaxed in the future.

All tables that you want to turn into a class need to have an integer primary key. That key should be defined like:

MySQL:
  `INT PRIMARY KEY AUTO_INCREMENT`
Postgres:
  `SERIAL PRIMARY KEY`
SQLite:
  `INTEGER PRIMARY KEY AUTOINCREMENT`

SQLObject does not support primary keys made up of multiple columns (that probably won’t change). It does not generally support tables with primary keys with business meaning – i.e., primary keys are assumed to be immutable (that won’t change).

At the moment foreign key column names must end in `"ID"` (case-insensitive). This restriction will probably be removed in the next release.

#### Workaround for primary keys made up of multiple columns

If the database table/view has ONE NUMERIC Primary Key then sqlmeta - idName should be used to map the table column name to SQLObject id column.

If the Primary Key consists only of number columns it is possible to create a virtual column `id` this way:

Example for Postgresql:

> select ‘1’||lpad(PK1,max_length_of_PK1,’0’)||lpad(PK2,max_length_of_PK2,’0’)||…||lpad(PKn,max_length_of_PKn,’0’) as “id”, column_PK1, column_PK2, .., column_PKn, column… from table;

Note:

- The arbitrary ‘1’ at the beginning of the string to allow for leading zeros of the first PK.
- The application designer has to determine the maximum length of each Primary Key.

This statement can be saved as a view or the column can be added to the database table, where it can be kept up to date with a database trigger.

Obviously the “view” method does generally not allow insert, updates or deletes. For Postgresql you may want to consult the chapter “RULES” for manipulating underlying tables.

For an alphanumeric Primary Key column a similar method is possible:

Every character of the lpaded PK has to be transfered using ascii(character) which returns a 3digit number which can be concatenated as shown above.

Caveats:

- this way the `id` may become a very large integer number which may cause troubles elsewhere.
- no performance loss takes place if the where clauses specifies the PK columns.

Example: CD-Album * Album: PK=ean * Tracks: PK=ean,disc_nr,track_nr

The database view to show the tracks starts:

> SELECT ean||lpad(“disc_nr”,2,’0’)||lpad(“track_nr”,2,’0’) as id, … Note: no leading ‘1’ and no padding necessary for ean numbers

Tracks.select(Tracks.q.ean==id) … where id is the ean of the Album.

### Changing the Naming Style

By default names in SQLObject are expected to be mixed case in Python (like `mixedCase`), and underscore-separated in SQL (like `mixed_case`). This applies to table and column names. The primary key is assumed to be simply `id`.

Other styles exist. A typical one is mixed case column names, and a primary key that includes the table name, like `ProductID`. You can use a different Style object to indicate a different naming convention. For instance:

    class Person(SQLObject):
        class sqlmeta:
            style = MixedCaseStyle(longID=True)
        firstName = StringCol()
        lastName = StringCol()

If you use `Person.createTable()`, you’ll get:

    CREATE TABLE Person (
        PersonID INT PRIMARY KEY,
        FirstName Text,
        LastName Text
    )

The MixedCaseStyle object handles the initial capitalization of words, but otherwise leaves them be. By using `longID=True`, we indicate that the primary key should look like a normal reference (`PersonID` for MixedCaseStyle, or `person_id` for the default style).

If you wish to change the style globally, assign the style to the connection, like:

    __connection__.style = MixedCaseStyle(longID=True)

### Irregular Naming

This is now covered in the Class sqlmeta section.

### Non-Integer Keys

While not strictly a legacy database issue, this fits into the category of “irregularities”. If you use non-integer keys, all primary key management is up to you. You must create the table yourself (SQLObject can create tables with int or str IDs), and when you create instances you must pass a `id` keyword argument into constructor (like `Person(id='555-55-5555', ...)`).

## DBConnection: Database Connections

The DBConnection module currently has six external classes, MySQLConnection, PostgresConnection, SQLiteConnection, SybaseConnection, MaxdbConnection, MSSQLConnection.

You can pass the keyword argument debug to any connector. If set to true, then any SQL sent to the database will also be printed to the console.

You can additionally pass logger keyword argument which should be a name of the logger to use. If specified and debug is `True`, SQLObject will write debug print statements via that logger instead of printing directly to console. The argument loglevel allows to choose the logging level - it can be `debug`, `info`, `warning`, `error`, `critical` or `exception`. In case logger is absent or empty SQLObject uses `print`’s instead of logging; loglevel can be `stdout` or `stderr` in this case; default is `stdout`.

To configure logging one can do something like that:

    import logging
    logging.basicConfig(
        filename='test.log',
        format='[%(asctime)s] %(name)s %(levelname)s: %(message)s',
        level=logging.DEBUG,
    )
    log = logging.getLogger("TEST")
    log.info("Log started")

    __connection__ = "sqlite:/:memory:?debug=1&logger=TEST&loglevel=debug"

The code redirects SQLObject debug messages to test.log file.

### MySQL

MySQLConnection takes the keyword arguments host, port, db, user, and password, just like MySQLdb.connect does.

MySQLConnection supports all the features, though MySQL only supports transactions when using the InnoDB backend; SQLObject can explicitly define the backend using `sqlmeta.createSQL`.

Supported drivers are `mysqldb`, `connector`, `oursql`, `pymysql` and `mariadb`, `pyodbc`, `pypyodbc` or `odbc` (try `pyodbc` and `pypyodbc`); defualt is `mysqldb`.

Keyword argument `conv` allows to pass a list of custom converters. Example:

    import time
    import sqlobject
    import MySQLdb.converters

    def _mysql_timestamp_converter(raw):
             """Convert a MySQL TIMESTAMP to a floating point number representing
             the seconds since the Un*x Epoch. It uses custom code the input seems
             to be the new (MySQL 4.1+) timestamp format, otherwise code from the
             MySQLdb module is used."""
             if raw[4] == '-':
                 return time.mktime(time.strptime(raw, '%Y-%m-%d %H:%M:%S'))
             else:
                 return MySQLdb.converters.mysql_timestamp_converter(raw)

    conversions = MySQLdb.converters.conversions.copy()
    conversions[MySQLdb.constants.FIELD_TYPE.TIMESTAMP] = _mysql_timestamp_converter

    MySQLConnection = sqlobject.mysql.builder()
    connection = MySQLConnection(user='foo', db='somedb', conv=conversions)

Connection-specific parameters are: `unix_socket`, `init_command`, `read_default_file`, `read_default_group`, `conv`, `connect_timeout`, `compress`, `named_pipe`, `use_unicode`, `client_flag`, `local_infile`, `ssl_key`, `ssl_cert`, `ssl_ca`, `ssl_capath`, `charset`.

### Postgres

PostgresConnection takes a single connection string, like `"dbname=something user=some_user"`, just like psycopg.connect. You can also use the same keyword arguments as for MySQLConnection, and a dsn string will be constructed.

PostgresConnection supports transactions and all other features.

The user can choose a DB API driver for PostgreSQL by using a `driver` parameter in DB URI or PostgresConnection that can be a comma-separated list of driver names. Possible drivers are: `psycopg2`, `psycopg` (alias for `psycopg2`), `pygresql`, `pypostgresql`, `pg8000`, `pyodbc`, `pypyodbc` or `odbc` (try `pyodbc` and `pypyodbc`). Default is `psycopg`.

Connection-specific parameters are: `sslmode`, `unicodeCols`, `schema`, `charset`.

### SQLite

SQLiteConnection takes the a single string, which is the path to the database file.

SQLite puts all data into one file, with a journal file that is opened in the same directory during operation (the file is deleted when the program quits). SQLite does not restrict the types you can put in a column – strings can go in integer columns, dates in integers, etc.

SQLite may have concurrency issues, depending on your usage in a multi-threaded environment.

The user can choose a DB API driver for SQLite by using a `driver` parameter in DB URI or SQLiteConnection that can be a comma-separated list of driver names. Possible drivers are: `pysqlite2` (alias `sqlite2`), `sqlite3`, `sqlite` (alias `sqlite1`), `supersqlite`. Default is to test supersqlite, pysqlite2, sqlite3 and sqlite in that order.

Connection-specific parameters are: `encoding`, `mode`, `timeout`, `check_same_thread`, `use_table_info`.

### Firebird

FirebirdConnection takes the arguments host, db, user (default `"sysdba"`), password (default `"masterkey"`).

Firebird supports all the features. Support is still young, so there may be some issues, especially with concurrent access, and especially using lazy selects. Try `list(MyClass.select())` to avoid concurrent cursors if you have problems (using `list()` will pre-fetch all the results of a select).

Firebird support `fdb`, `kinterbasdb` or `firebirdsql` drivers. Default are `fdb` and `kinterbasdb`.

There could be a problem if one tries to connect to a server running on w32 from a program running on Unix; the problem is how to specify the database so that SQLObject correctly parses it. Vertical bar is replaces by a semicolon only on a w32. On Unix a vertical bar is a pretty normal character and must not be processed.

The most correct way to fix the problem is to connect to the DB using a database name, not a file name. In the Firebird a DBA can set an alias instead of database name in the aliases.conf file

Example from Firebird 2.0 Administrators Manual:

    # fbdb1 is on a Windows server:
    fbdb1 = c:\Firebird\sample\Employee.fdb

Now a program can connect to firebird://host:port/fbdb1.

One can edit aliases.conf whilst the server is running. There is no need to stop and restart the server in order for new aliases.conf entries to be recognised.

If you are using indexes and get an error like key size exceeds implementation restriction for index, see this page to understand the restrictions on your indexing.

Connection-specific parameters are: `dialect`, `role`, `charset`.

### Sybase

SybaseConnection takes the arguments host, db, user, and password. It also takes the extra boolean argument locking (default True), which is passed through when performing a connection. You may use a False value for locking if you are not using multiple threads, for a slight performance boost.

It uses the Sybase module.

Connection-specific parameters are: `locking`, `autoCommit`.

### MAX DB

MAX DB, also known as SAP DB, is available from a partnership of SAP and MySQL. It takes the typical arguments: host, database, user, password. It also takes the arguments sqlmode (default `"internal"`), isolation, and timeout, which are passed through when creating the connection to the database.

It uses the sapdb module.

Connection-specific parameters are: `autoCommit`, `sqlmode`, `isolation`, `timeout`.

### MS SQL Server

The MSSQLConnection objects wants to use new style connection strings in the format of

mssql://user:pass@host:port/db

This will then be mapped to either the correct driver format. If running SQL Server on a “named” port, make sure to specify the port number in the URI.

The two drivers currently supported are adodbapi and pymssql.

The user can choose a DB API driver for MSSQL by using a `driver` parameter in DB URI or MSSQLConnection that can be a comma-separated list of driver names. Possible drivers are: `adodb` (alias `adodbapi`) and `pymssql`. Default is to test `adodbapi` and `pymssql` in that order.

Connection-specific parameters are: `autoCommit`, `timeout`.

## Events (signals)

Signals are a mechanism to be notified when data or schema changes happen through SQLObject. This may be useful for doing custom data validation, logging changes, setting default attributes, etc. Some of what signals can do is also possible by overriding methods, but signals may provide a cleaner way, especially across classes not related by inheritance.

Example:

    from sqlobject.events import listen, RowUpdateSignal, RowCreatedSignal
    from model import Users

    def update_listener(instance, kwargs):
        """keep "last_updated" field current"""
        import datetime
        # BAD method 1, causes infinite recursion?
        # instance should be read-only
        instance.last_updated = datetime.datetime.now()
        # GOOD method 2
        kwargs['last_updated'] = datetime.datetime.now()

    def created_listener(instance, kwargs, post_funcs):
        """"email me when new users added"""
        # email() implementation left as an exercise for the reader
        msg = "%s just was just added to the database!" % kwargs['name']
        email(msg)

    listen(update_listener, Users, RowUpdateSignal)
    listen(created_listener, Users, RowCreatedSignal)

## Exported Symbols

You can use `from sqlobject import *`, though you don’t have to. It exports a minimal number of symbols. The symbols exported:

From sqlobject.main:

- NoDefault
- SQLObject
- getID
- getObject

From sqlobject.col: * Col * StringCol * IntCol * FloatCol * KeyCol * ForeignKey * EnumCol * SetCol * DateTimeCol * DateCol * TimeCol * TimestampCol * DecimalCol * CurrencyCol

From sqlobject.joins: * MultipleJoin * RelatedJoin

From sqlobject.styles: * Style * MixedCaseUnderscoreStyle * DefaultStyle * MixedCaseStyle

From sqlobject.sqlbuilder:

- AND
- OR
- NOT
- IN
- LIKE
- DESC
- CONTAINSSTRING
- const
- func

### LEFT JOIN and other JOINs

First look in the FAQ, question “How can I do a LEFT JOIN?”

Still here? Well. To perform a JOIN use one of the JOIN helpers from SQLBuilder. Pass an instance of the helper to .select() method. For example:

    from sqlobject.sqlbuilder import LEFTJOINOn
    MyTable.select(
        join=LEFTJOINOn(Table1, Table2,
                        Table1.q.name == Table2.q.value))

will generate the query:

    SELECT my_table.* FROM my_table, table1
    LEFT JOIN table2 ON table1.name = table2.value;

If you want to join with the primary table - leave the first table None:

    MyTable.select(
        join=LEFTJOINOn(None, Table1,
                        MyTable.q.name == Table1.q.value))

will generate the query:

    SELECT my_table.* FROM my_table
    LEFT JOIN table2 ON my_table.name = table1.value;

The join argument for .select() can be a JOIN() or a sequence (list/tuple) of JOIN()s.

Available joins are JOIN, INNERJOIN, CROSSJOIN, STRAIGHTJOIN, LEFTJOIN, LEFTOUTERJOIN, NATURALJOIN, NATURALLEFTJOIN, NATURALLEFTOUTERJOIN, RIGHTJOIN, RIGHTOUTERJOIN, NATURALRIGHTJOIN, NATURALRIGHTOUTERJOIN, FULLJOIN, FULLOUTERJOIN, NATURALFULLJOIN, NATURALFULLOUTERJOIN, INNERJOINOn, LEFTJOINOn, LEFTOUTERJOINOn, RIGHTJOINOn, RIGHTOUTERJOINOn, FULLJOINOn, FULLOUTERJOINOn, INNERJOINUsing, LEFTJOINUsing, LEFTOUTERJOINUsing, RIGHTJOINUsing, RIGHTOUTERJOINUsing, FULLJOINUsing, FULLOUTERJOINUsing.

### How can I join a table with itself?

Use Alias from SQLBuilder. Example:

    from sqlobject.sqlbuilder import Alias
    alias = Alias(MyTable, "my_table_alias")
    MyTable.select(MyTable.q.name == alias.q.value)

will generate the query:

    SELECT my_table.* FROM my_table, my_table AS my_table_alias
    WHERE my_table.name = my_table_alias.value;

### Can I use a JOIN() with aliases?

Sure! That’s a situation the JOINs and aliases were primary developed for. Code:

    from sqlobject.sqlbuilder import LEFTJOINOn, Alias
    alias = Alias(OtherTable, "other_table_alias")
    MyTable.select(MyTable.q.name == OtherTable.q.value,
        join=LEFTJOINOn(MyTable, alias, MyTable.col1 == alias.q.col2))

will result in the query:

    SELECT my_table.* FROM other_table,
        my_table LEFT JOIN other_table AS other_table_alias
    WHERE my_table.name == other_table.value AND
        my_table.col1 = other_table_alias.col2.

### Subqueries (subselects)

You can run queries with subqueries (subselects) on those DBMS that can do subqueries (MySQL supports subqueries from version 4.1).

Use corresponding classes and functions from SQLBuilder:

    from sqlobject.sqlbuilder import EXISTS, Select
    select = Test1.select(EXISTS(Select(Test2.q.col2, where=(Outer(Test1).q.col1 == Test2.q.col2))))

generates the query:

    SELECT test1.id, test1.col1 FROM test1 WHERE
    EXISTS (SELECT test2.col2 FROM test2 WHERE (test1.col1 = test2.col2))

Note the usage of Outer - it is a helper to allow referring to a table in the outer query.

Select() is used instead of .select() because you need to control what columns the inner query returns.

Available queries are `IN()`, `NOTIN()`, `EXISTS()`, `NOTEXISTS()`, `SOME()`, `ANY()` and `ALL()`. The last 3 are used with comparison operators, like this: `somevalue = ANY(Select(...))`.

### Utilities

Some useful utility functions are included with SQLObject. For more information see their module docstrings.

- sqlobject.util.csvexport

### SQLBuilder

For more information on SQLBuilder, read the SQLBuilder Documentation.