# SQLAlchemy 0.8 Documentation

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Release: 0.8.2 | Release Date: July 3, 2013

# Declarative

## Synopsis

SQLAlchemy object-relational configuration involves the combination of `Table`, `mapper()`, and class objects to define a mapped class. `declarative` allows all three to be expressed at once within the class declaration. As much as possible, regular SQLAlchemy schema and ORM constructs are used directly, so that configuration between “classical” ORM usage and declarative remain highly similar.

As a simple example:

    from sqlalchemy.ext.declarative import declarative_base

    Base = declarative_base()

    class SomeClass(Base):
        __tablename__ = 'some_table'
        id = Column(Integer, primary_key=True)
        name =  Column(String(50))

Above, the `declarative_base()` callable returns a new base class from which all mapped classes should inherit. When the class definition is completed, a new `Table` and `mapper()` will have been generated.

The resulting table and mapper are accessible via `__table__` and `__mapper__` attributes on the `SomeClass` class:

    # access the mapped Table
    SomeClass.__table__

    # access the Mapper
    SomeClass.__mapper__

## Defining Attributes

In the previous example, the `Column` objects are automatically named with the name of the attribute to which they are assigned.

To name columns explicitly with a name distinct from their mapped attribute, just give the column a name. Below, column “some_table_id” is mapped to the “id” attribute of SomeClass, but in SQL will be represented as “some_table_id”:

    class SomeClass(Base):
        __tablename__ = 'some_table'
        id = Column("some_table_id", Integer, primary_key=True)

Attributes may be added to the class after its construction, and they will be added to the underlying `Table` and `mapper()` definitions as appropriate:

    SomeClass.data = Column('data', Unicode)
    SomeClass.related = relationship(RelatedInfo)

Classes which are constructed using declarative can interact freely with classes that are mapped explicitly with `mapper()`.

It is recommended, though not required, that all tables share the same underlying `MetaData` object, so that string-configured `ForeignKey` references can be resolved without issue.

## Accessing the MetaData

The `declarative_base()` base class contains a `MetaData` object where newly defined `Table` objects are collected. This object is intended to be accessed directly for `MetaData`-specific operations. Such as, to issue CREATE statements for all tables:

    engine = create_engine('sqlite://')
    Base.metadata.create_all(engine)

`declarative_base()` can also receive a pre-existing `MetaData` object, which allows a declarative setup to be associated with an already existing traditional collection of `Table` objects:

    mymetadata = MetaData()
    Base = declarative_base(metadata=mymetadata)

## Configuring Relationships

Relationships to other classes are done in the usual way, with the added feature that the class specified to `relationship()` may be a string name. The “class registry” associated with `Base` is used at mapper compilation time to resolve the name into the actual class object, which is expected to have been defined once the mapper configuration is used:

    class User(Base):
        __tablename__ = 'users'

        id = Column(Integer, primary_key=True)
        name = Column(String(50))
        addresses = relationship("Address", backref="user")

    class Address(Base):
        __tablename__ = 'addresses'

        id = Column(Integer, primary_key=True)
        email = Column(String(50))
        user_id = Column(Integer, ForeignKey('users.id'))

Column constructs, since they are just that, are immediately usable, as below where we define a primary join condition on the `Address` class using them:

    class Address(Base):
        __tablename__ = 'addresses'

        id = Column(Integer, primary_key=True)
        email = Column(String(50))
        user_id = Column(Integer, ForeignKey('users.id'))
        user = relationship(User, primaryjoin=user_id == User.id)

In addition to the main argument for `relationship()`, other arguments which depend upon the columns present on an as-yet undefined class may also be specified as strings. These strings are evaluated as Python expressions. The full namespace available within this evaluation includes all classes mapped for this declarative base, as well as the contents of the `sqlalchemy` package, including expression functions like `desc()` and `func`:

    class User(Base):
        # ....
        addresses = relationship("Address",
                             order_by="desc(Address.email)",
                             primaryjoin="Address.user_id==User.id")

For the case where more than one module contains a class of the same name, string class names can also be specified as module-qualified paths within any of these string expressions:

    class User(Base):
        # ....
        addresses = relationship("myapp.model.address.Address",
                             order_by="desc(myapp.model.address.Address.email)",
                             primaryjoin="myapp.model.address.Address.user_id=="
                                            "myapp.model.user.User.id")

The qualified path can be any partial path that removes ambiguity between the names. For example, to disambiguate between `myapp.model.address.Address` and `myapp.model.lookup.Address`, we can specify `address.Address` or `lookup.Address`:

    class User(Base):
        # ....
        addresses = relationship("address.Address",
                             order_by="desc(address.Address.email)",
                             primaryjoin="address.Address.user_id=="
                                            "User.id")

New in version 0.8: module-qualified paths can be used when specifying string arguments with Declarative, in order to specify specific modules.

Two alternatives also exist to using string-based attributes. A lambda can also be used, which will be evaluated after all mappers have been configured:

    class User(Base):
        # ...
        addresses = relationship(lambda: Address,
                             order_by=lambda: desc(Address.email),
                             primaryjoin=lambda: Address.user_id==User.id)

Or, the relationship can be added to the class explicitly after the classes are available:

    User.addresses = relationship(Address,
                              primaryjoin=Address.user_id==User.id)

## Configuring Many-to-Many Relationships

Many-to-many relationships are also declared in the same way with declarative as with traditional mappings. The `secondary` argument to `relationship()` is as usual passed a `Table` object, which is typically declared in the traditional way. The `Table` usually shares the `MetaData` object used by the declarative base:

    keywords = Table(
        'keywords', Base.metadata,
        Column('author_id', Integer, ForeignKey('authors.id')),
        Column('keyword_id', Integer, ForeignKey('keywords.id'))
        )

    class Author(Base):
        __tablename__ = 'authors'
        id = Column(Integer, primary_key=True)
        keywords = relationship("Keyword", secondary=keywords)

Like other `relationship()` arguments, a string is accepted as well, passing the string name of the table as defined in the `Base.metadata.tables` collection:

    class Author(Base):
        __tablename__ = 'authors'
        id = Column(Integer, primary_key=True)
        keywords = relationship("Keyword", secondary="keywords")

As with traditional mapping, its generally not a good idea to use a `Table` as the “secondary” argument which is also mapped to a class, unless the `relationship()` is declared with `viewonly=True`. Otherwise, the unit-of-work system may attempt duplicate INSERT and DELETE statements against the underlying table.

## Defining SQL Expressions

See SQL Expressions as Mapped Attributes for examples on declaratively mapping attributes to SQL expressions.

## Table Configuration

Table arguments other than the name, metadata, and mapped Column arguments are specified using the `__table_args__` class attribute. This attribute accommodates both positional as well as keyword arguments that are normally sent to the `Table` constructor. The attribute can be specified in one of two forms. One is as a dictionary:

    class MyClass(Base):
        __tablename__ = 'sometable'
        __table_args__ = {'mysql_engine':'InnoDB'}

The other, a tuple, where each argument is positional (usually constraints):

    class MyClass(Base):
        __tablename__ = 'sometable'
        __table_args__ = (
                ForeignKeyConstraint(['id'], ['remote_table.id']),
                UniqueConstraint('foo'),
                )

Keyword arguments can be specified with the above form by specifying the last argument as a dictionary:

    class MyClass(Base):
        __tablename__ = 'sometable'
        __table_args__ = (
                ForeignKeyConstraint(['id'], ['remote_table.id']),
                UniqueConstraint('foo'),
                {'autoload':True}
                )

## Using a Hybrid Approach with __table__

As an alternative to `__tablename__`, a direct `Table` construct may be used. The `Column` objects, which in this case require their names, will be added to the mapping just like a regular mapping to a table:

    class MyClass(Base):
        __table__ = Table('my_table', Base.metadata,
            Column('id', Integer, primary_key=True),
            Column('name', String(50))
        )

`__table__` provides a more focused point of control for establishing table metadata, while still getting most of the benefits of using declarative. An application that uses reflection might want to load table metadata elsewhere and pass it to declarative classes:

    from sqlalchemy.ext.declarative import declarative_base

    Base = declarative_base()
    Base.metadata.reflect(some_engine)

    class User(Base):
        __table__ = metadata.tables['user']

    class Address(Base):
        __table__ = metadata.tables['address']

Some configuration schemes may find it more appropriate to use `__table__`, such as those which already take advantage of the data-driven nature of `Table` to customize and/or automate schema definition.

Note that when the `__table__` approach is used, the object is immediately usable as a plain `Table` within the class declaration body itself, as a Python class is only another syntactical block. Below this is illustrated by using the `id` column in the `primaryjoin` condition of a `relationship()`:

    class MyClass(Base):
        __table__ = Table('my_table', Base.metadata,
            Column('id', Integer, primary_key=True),
            Column('name', String(50))
        )

        widgets = relationship(Widget,
                    primaryjoin=Widget.myclass_id==__table__.c.id)

Similarly, mapped attributes which refer to `__table__` can be placed inline, as below where we assign the `name` column to the attribute `_name`, generating a synonym for `name`:

    from sqlalchemy.ext.declarative import synonym_for

    class MyClass(Base):
        __table__ = Table('my_table', Base.metadata,
            Column('id', Integer, primary_key=True),
            Column('name', String(50))
        )

        _name = __table__.c.name

        @synonym_for("_name")
        def name(self):
            return "Name: %s" % _name

## Using Reflection with Declarative

It’s easy to set up a `Table` that uses `autoload=True` in conjunction with a mapped class:

    class MyClass(Base):
        __table__ = Table('mytable', Base.metadata,
                        autoload=True, autoload_with=some_engine)

However, one improvement that can be made here is to not require the `Engine` to be available when classes are being first declared. To achieve this, use the `DeferredReflection` mixin, which sets up mappings only after a special `prepare(engine)` step is called:

    from sqlalchemy.ext.declarative import declarative_base, DeferredReflection

    Base = declarative_base(cls=DeferredReflection)

    class Foo(Base):
        __tablename__ = 'foo'
        bars = relationship("Bar")

    class Bar(Base):
        __tablename__ = 'bar'

        # illustrate overriding of "bar.foo_id" to have
        # a foreign key constraint otherwise not
        # reflected, such as when using MySQL
        foo_id = Column(Integer, ForeignKey('foo.id'))

    Base.prepare(e)

New in version 0.8: Added `DeferredReflection`.

## Mapper Configuration

Declarative makes use of the `mapper()` function internally when it creates the mapping to the declared table. The options for `mapper()` are passed directly through via the `__mapper_args__` class attribute. As always, arguments which reference locally mapped columns can reference them directly from within the class declaration:

    from datetime import datetime

    class Widget(Base):
        __tablename__ = 'widgets'

        id = Column(Integer, primary_key=True)
        timestamp = Column(DateTime, nullable=False)

        __mapper_args__ = {
                        'version_id_col': timestamp,
                        'version_id_generator': lambda v:datetime.now()
                    }

## Inheritance Configuration

Declarative supports all three forms of inheritance as intuitively as possible. The `inherits` mapper keyword argument is not needed as declarative will determine this from the class itself. The various “polymorphic” keyword arguments are specified using `__mapper_args__`.

### Joined Table Inheritance

Joined table inheritance is defined as a subclass that defines its own table:

    class Person(Base):
        __tablename__ = 'people'
        id = Column(Integer, primary_key=True)
        discriminator = Column('type', String(50))
        __mapper_args__ = {'polymorphic_on': discriminator}

    class Engineer(Person):
        __tablename__ = 'engineers'
        __mapper_args__ = {'polymorphic_identity': 'engineer'}
        id = Column(Integer, ForeignKey('people.id'), primary_key=True)
        primary_language = Column(String(50))

Note that above, the `Engineer.id` attribute, since it shares the same attribute name as the `Person.id` attribute, will in fact represent the `people.id` and `engineers.id` columns together, with the “Engineer.id” column taking precedence if queried directly. To provide the `Engineer` class with an attribute that represents only the `engineers.id` column, give it a different attribute name:

    class Engineer(Person):
        __tablename__ = 'engineers'
        __mapper_args__ = {'polymorphic_identity': 'engineer'}
        engineer_id = Column('id', Integer, ForeignKey('people.id'),
                                                    primary_key=True)
        primary_language = Column(String(50))

Changed in version 0.7: joined table inheritance favors the subclass column over that of the superclass, such as querying above for `Engineer.id`. Prior to 0.7 this was the reverse.

### Single Table Inheritance

Single table inheritance is defined as a subclass that does not have its own table; you just leave out the `__table__` and `__tablename__` attributes:

    class Person(Base):
        __tablename__ = 'people'
        id = Column(Integer, primary_key=True)
        discriminator = Column('type', String(50))
        __mapper_args__ = {'polymorphic_on': discriminator}

    class Engineer(Person):
        __mapper_args__ = {'polymorphic_identity': 'engineer'}
        primary_language = Column(String(50))

When the above mappers are configured, the `Person` class is mapped to the `people` table before the `primary_language` column is defined, and this column will not be included in its own mapping. When `Engineer` then defines the `primary_language` column, the column is added to the `people` table so that it is included in the mapping for `Engineer` and is also part of the table’s full set of columns. Columns which are not mapped to `Person` are also excluded from any other single or joined inheriting classes using the `exclude_properties` mapper argument. Below, `Manager` will have all the attributes of `Person` and `Manager` but not the `primary_language` attribute of `Engineer`:

    class Manager(Person):
        __mapper_args__ = {'polymorphic_identity': 'manager'}
        golf_swing = Column(String(50))

The attribute exclusion logic is provided by the `exclude_properties` mapper argument, and declarative’s default behavior can be disabled by passing an explicit `exclude_properties` collection (empty or otherwise) to the `__mapper_args__`.

#### Resolving Column Conflicts

Note above that the `primary_language` and `golf_swing` columns are “moved up” to be applied to `Person.__table__`, as a result of their declaration on a subclass that has no table of its own. A tricky case comes up when two subclasses want to specify the same column, as below:

    class Person(Base):
        __tablename__ = 'people'
        id = Column(Integer, primary_key=True)
        discriminator = Column('type', String(50))
        __mapper_args__ = {'polymorphic_on': discriminator}

    class Engineer(Person):
        __mapper_args__ = {'polymorphic_identity': 'engineer'}
        start_date = Column(DateTime)

    class Manager(Person):
        __mapper_args__ = {'polymorphic_identity': 'manager'}
        start_date = Column(DateTime)

Above, the `start_date` column declared on both `Engineer` and `Manager` will result in an error:

    sqlalchemy.exc.ArgumentError: Column 'start_date' on class
    <class '__main__.Manager'> conflicts with existing
    column 'people.start_date'

In a situation like this, Declarative can’t be sure of the intent, especially if the `start_date` columns had, for example, different types. A situation like this can be resolved by using `declared_attr` to define the `Column` conditionally, taking care to return the existing column via the parent `__table__` if it already exists:

    from sqlalchemy.ext.declarative import declared_attr

    class Person(Base):
        __tablename__ = 'people'
        id = Column(Integer, primary_key=True)
        discriminator = Column('type', String(50))
        __mapper_args__ = {'polymorphic_on': discriminator}

    class Engineer(Person):
        __mapper_args__ = {'polymorphic_identity': 'engineer'}

        @declared_attr
        def start_date(cls):
            "Start date column, if not present already."
            return Person.__table__.c.get('start_date', Column(DateTime))

    class Manager(Person):
        __mapper_args__ = {'polymorphic_identity': 'manager'}

        @declared_attr
        def start_date(cls):
            "Start date column, if not present already."
            return Person.__table__.c.get('start_date', Column(DateTime))

Above, when `Manager` is mapped, the `start_date` column is already present on the `Person` class. Declarative lets us return that `Column` as a result in this case, where it knows to skip re-assigning the same column. If the mapping is mis-configured such that the `start_date` column is accidentally re-assigned to a different table (such as, if we changed `Manager` to be joined inheritance without fixing `start_date`), an error is raised which indicates an existing `Column` is trying to be re-assigned to a different owning `Table`.

New in version 0.8: `declared_attr` can be used on a non-mixin class, and the returned `Column` or other mapped attribute will be applied to the mapping as any other attribute. Previously, the resulting attribute would be ignored, and also result in a warning being emitted when a subclass was created.

New in version 0.8: `declared_attr`, when used either with a mixin or non-mixin declarative class, can return an existing `Column` already assigned to the parent `Table`, to indicate that the re-assignment of the `Column` should be skipped, however should still be mapped on the target class, in order to resolve duplicate column conflicts.

The same concept can be used with mixin classes (see Mixin and Custom Base Classes):

    class Person(Base):
        __tablename__ = 'people'
        id = Column(Integer, primary_key=True)
        discriminator = Column('type', String(50))
        __mapper_args__ = {'polymorphic_on': discriminator}

    class HasStartDate(object):
        @declared_attr
        def start_date(cls):
            return cls.__table__.c.get('start_date', Column(DateTime))

    class Engineer(HasStartDate, Person):
        __mapper_args__ = {'polymorphic_identity': 'engineer'}

    class Manager(HasStartDate, Person):
        __mapper_args__ = {'polymorphic_identity': 'manager'}

The above mixin checks the local `__table__` attribute for the column. Because we’re using single table inheritance, we’re sure that in this case, `cls.__table__` refers to `People.__table__`. If we were mixing joined- and single-table inheritance, we might want our mixin to check more carefully if `cls.__table__` is really the `Table` we’re looking for.

### Concrete Table Inheritance

Concrete is defined as a subclass which has its own table and sets the `concrete` keyword argument to `True`:

    class Person(Base):
        __tablename__ = 'people'
        id = Column(Integer, primary_key=True)
        name = Column(String(50))

    class Engineer(Person):
        __tablename__ = 'engineers'
        __mapper_args__ = {'concrete':True}
        id = Column(Integer, primary_key=True)
        primary_language = Column(String(50))
        name = Column(String(50))

Usage of an abstract base class is a little less straightforward as it requires usage of `polymorphic_union()`, which needs to be created with the `Table` objects before the class is built:

    engineers = Table('engineers', Base.metadata,
                    Column('id', Integer, primary_key=True),
                    Column('name', String(50)),
                    Column('primary_language', String(50))
                )
    managers = Table('managers', Base.metadata,
                    Column('id', Integer, primary_key=True),
                    Column('name', String(50)),
                    Column('golf_swing', String(50))
                )

    punion = polymorphic_union({
        'engineer':engineers,
        'manager':managers
    }, 'type', 'punion')

    class Person(Base):
        __table__ = punion
        __mapper_args__ = {'polymorphic_on':punion.c.type}

    class Engineer(Person):
        __table__ = engineers
        __mapper_args__ = {'polymorphic_identity':'engineer', 'concrete':True}

    class Manager(Person):
        __table__ = managers
        __mapper_args__ = {'polymorphic_identity':'manager', 'concrete':True}

#### Using the Concrete Helpers

Helper classes provides a simpler pattern for concrete inheritance. With these objects, the `__declare_last__` helper is used to configure the “polymorphic” loader for the mapper after all subclasses have been declared.

New in version 0.7.3.

An abstract base can be declared using the `AbstractConcreteBase` class:

    from sqlalchemy.ext.declarative import AbstractConcreteBase

    class Employee(AbstractConcreteBase, Base):
        pass

To have a concrete `employee` table, use `ConcreteBase` instead:

    from sqlalchemy.ext.declarative import ConcreteBase

    class Employee(ConcreteBase, Base):
        __tablename__ = 'employee'
        employee_id = Column(Integer, primary_key=True)
        name = Column(String(50))
        __mapper_args__ = {
                        'polymorphic_identity':'employee',
                        'concrete':True}

Either `Employee` base can be used in the normal fashion:

    class Manager(Employee):
        __tablename__ = 'manager'
        employee_id = Column(Integer, primary_key=True)
        name = Column(String(50))
        manager_data = Column(String(40))
        __mapper_args__ = {
                        'polymorphic_identity':'manager',
                        'concrete':True}

    class Engineer(Employee):
        __tablename__ = 'engineer'
        employee_id = Column(Integer, primary_key=True)
        name = Column(String(50))
        engineer_info = Column(String(40))
        __mapper_args__ = {'polymorphic_identity':'engineer',
                        'concrete':True}

## Mixin and Custom Base Classes

A common need when using `declarative` is to share some functionality, such as a set of common columns, some common table options, or other mapped properties, across many classes. The standard Python idioms for this is to have the classes inherit from a base which includes these common features.

When using `declarative`, this idiom is allowed via the usage of a custom declarative base class, as well as a “mixin” class which is inherited from in addition to the primary base. Declarative includes several helper features to make this work in terms of how mappings are declared. An example of some commonly mixed-in idioms is below:

    from sqlalchemy.ext.declarative import declared_attr

    class MyMixin(object):

        @declared_attr
        def __tablename__(cls):
            return cls.__name__.lower()

        __table_args__ = {'mysql_engine': 'InnoDB'}
        __mapper_args__= {'always_refresh': True}

        id =  Column(Integer, primary_key=True)

    class MyModel(MyMixin, Base):
        name = Column(String(1000))

Where above, the class `MyModel` will contain an “id” column as the primary key, a `__tablename__` attribute that derives from the name of the class itself, as well as `__table_args__` and `__mapper_args__` defined by the `MyMixin` mixin class.

There’s no fixed convention over whether `MyMixin` precedes `Base` or not. Normal Python method resolution rules apply, and the above example would work just as well with:

    class MyModel(Base, MyMixin):
        name = Column(String(1000))

This works because `Base` here doesn’t define any of the variables that `MyMixin` defines, i.e. `__tablename__`, `__table_args__`, `id`, etc. If the `Base` did define an attribute of the same name, the class placed first in the inherits list would determine which attribute is used on the newly defined class.

### Augmenting the Base

In addition to using a pure mixin, most of the techniques in this section can also be applied to the base class itself, for patterns that should apply to all classes derived from a particular base. This is achieved using the `cls` argument of the `declarative_base()` function:

    from sqlalchemy.ext.declarative import declared_attr

    class Base(object):
        @declared_attr
        def __tablename__(cls):
            return cls.__name__.lower()

        __table_args__ = {'mysql_engine': 'InnoDB'}

        id =  Column(Integer, primary_key=True)

    from sqlalchemy.ext.declarative import declarative_base

    Base = declarative_base(cls=Base)

    class MyModel(Base):
        name = Column(String(1000))

Where above, `MyModel` and all other classes that derive from `Base` will have a table name derived from the class name, an `id` primary key column, as well as the “InnoDB” engine for MySQL.

### Mixing in Columns

The most basic way to specify a column on a mixin is by simple declaration:

    class TimestampMixin(object):
        created_at = Column(DateTime, default=func.now())

    class MyModel(TimestampMixin, Base):
        __tablename__ = 'test'

        id =  Column(Integer, primary_key=True)
        name = Column(String(1000))

Where above, all declarative classes that include `TimestampMixin` will also have a column `created_at` that applies a timestamp to all row insertions.

Those familiar with the SQLAlchemy expression language know that the object identity of clause elements defines their role in a schema. Two `Table` objects `a` and `b` may both have a column called `id`, but the way these are differentiated is that `a.c.id` and `b.c.id` are two distinct Python objects, referencing their parent tables `a` and `b` respectively.

In the case of the mixin column, it seems that only one `Column` object is explicitly created, yet the ultimate `created_at` column above must exist as a distinct Python object for each separate destination class. To accomplish this, the declarative extension creates a copy of each `Column` object encountered on a class that is detected as a mixin.

This copy mechanism is limited to simple columns that have no foreign keys, as a `ForeignKey` itself contains references to columns which can’t be properly recreated at this level. For columns that have foreign keys, as well as for the variety of mapper-level constructs that require destination-explicit context, the `declared_attr` decorator is provided so that patterns common to many classes can be defined as callables:

    from sqlalchemy.ext.declarative import declared_attr

    class ReferenceAddressMixin(object):
        @declared_attr
        def address_id(cls):
            return Column(Integer, ForeignKey('address.id'))

    class User(ReferenceAddressMixin, Base):
        __tablename__ = 'user'
        id = Column(Integer, primary_key=True)

Where above, the `address_id` class-level callable is executed at the point at which the `User` class is constructed, and the declarative extension can use the resulting `Column` object as returned by the method without the need to copy it.

Changed in version >: 0.6.5 Rename 0.6.5 `sqlalchemy.util.classproperty` into `declared_attr`.

Columns generated by `declared_attr` can also be referenced by `__mapper_args__` to a limited degree, currently by `polymorphic_on` and `version_id_col`, by specifying the classdecorator itself into the dictionary - the declarative extension will resolve them at class construction time:

    class MyMixin:
        @declared_attr
        def type_(cls):
            return Column(String(50))

        __mapper_args__= {'polymorphic_on':type_}

    class MyModel(MyMixin, Base):
        __tablename__='test'
        id =  Column(Integer, primary_key=True)

### Mixing in Relationships

Relationships created by `relationship()` are provided with declarative mixin classes exclusively using the `declared_attr` approach, eliminating any ambiguity which could arise when copying a relationship and its possibly column-bound contents. Below is an example which combines a foreign key column and a relationship so that two classes `Foo` and `Bar` can both be configured to reference a common target class via many-to-one:

    class RefTargetMixin(object):
        @declared_attr
        def target_id(cls):
            return Column('target_id', ForeignKey('target.id'))

        @declared_attr
        def target(cls):
            return relationship("Target")

    class Foo(RefTargetMixin, Base):
        __tablename__ = 'foo'
        id = Column(Integer, primary_key=True)

    class Bar(RefTargetMixin, Base):
        __tablename__ = 'bar'
        id = Column(Integer, primary_key=True)

    class Target(Base):
        __tablename__ = 'target'
        id = Column(Integer, primary_key=True)

`relationship()` definitions which require explicit primaryjoin, order_by etc. expressions should use the string forms for these arguments, so that they are evaluated as late as possible. To reference the mixin class in these expressions, use the given `cls` to get its name:

    class RefTargetMixin(object):
        @declared_attr
        def target_id(cls):
            return Column('target_id', ForeignKey('target.id'))

        @declared_attr
        def target(cls):
            return relationship("Target",
                primaryjoin="Target.id==%s.target_id" % cls.__name__
            )

### Mixing in deferred(), column_property(), and other MapperProperty classes

Like `relationship()`, all `MapperProperty` subclasses such as `deferred()`, `column_property()`, etc. ultimately involve references to columns, and therefore, when used with declarative mixins, have the `declared_attr` requirement so that no reliance on copying is needed:

    class SomethingMixin(object):

        @declared_attr
        def dprop(cls):
            return deferred(Column(Integer))

    class Something(SomethingMixin, Base):
        __tablename__ = "something"

### Mixing in Association Proxy and Other Attributes

Mixins can specify user-defined attributes as well as other extension units such as `association_proxy()`. The usage of `declared_attr` is required in those cases where the attribute must be tailored specifically to the target subclass. An example is when constructing multiple `association_proxy()` attributes which each target a different type of child object. Below is an `association_proxy()` / mixin example which provides a scalar list of string values to an implementing class:

    from sqlalchemy import Column, Integer, ForeignKey, String
    from sqlalchemy.orm import relationship
    from sqlalchemy.ext.associationproxy import association_proxy
    from sqlalchemy.ext.declarative import declarative_base, declared_attr

    Base = declarative_base()

    class HasStringCollection(object):
        @declared_attr
        def _strings(cls):
            class StringAttribute(Base):
                __tablename__ = cls.string_table_name
                id = Column(Integer, primary_key=True)
                value = Column(String(50), nullable=False)
                parent_id = Column(Integer,
                                ForeignKey('%s.id' % cls.__tablename__),
                                nullable=False)
                def __init__(self, value):
                    self.value = value

            return relationship(StringAttribute)

        @declared_attr
        def strings(cls):
            return association_proxy('_strings', 'value')

    class TypeA(HasStringCollection, Base):
        __tablename__ = 'type_a'
        string_table_name = 'type_a_strings'
        id = Column(Integer(), primary_key=True)

    class TypeB(HasStringCollection, Base):
        __tablename__ = 'type_b'
        string_table_name = 'type_b_strings'
        id = Column(Integer(), primary_key=True)

Above, the `HasStringCollection` mixin produces a `relationship()` which refers to a newly generated class called `StringAttribute`. The `StringAttribute` class is generated with it’s own `Table` definition which is local to the parent class making usage of the `HasStringCollection` mixin. It also produces an `association_proxy()` object which proxies references to the `strings` attribute onto the `value` attribute of each `StringAttribute` instance.

`TypeA` or `TypeB` can be instantiated given the constructor argument `strings`, a list of strings:

    ta = TypeA(strings=['foo', 'bar'])
    tb = TypeA(strings=['bat', 'bar'])

This list will generate a collection of `StringAttribute` objects, which are persisted into a table that’s local to either the `type_a_strings` or `type_b_strings` table:

    >>> print ta._strings
    [<__main__.StringAttribute object at 0x10151cd90>,
        <__main__.StringAttribute object at 0x10151ce10>]

When constructing the `association_proxy()`, the `declared_attr` decorator must be used so that a distinct `association_proxy()` object is created for each of the `TypeA` and `TypeB` classes.

New in version 0.8: `declared_attr` is usable with non-mapped attributes, including user-defined attributes as well as `association_proxy()`.

### Controlling table inheritance with mixins

The `__tablename__` attribute in conjunction with the hierarchy of classes involved in a declarative mixin scenario controls what type of table inheritance, if any, is configured by the declarative extension.

If the `__tablename__` is computed by a mixin, you may need to control which classes get the computed attribute in order to get the type of table inheritance you require.

For example, if you had a mixin that computes `__tablename__` but where you wanted to use that mixin in a single table inheritance hierarchy, you can explicitly specify `__tablename__` as `None` to indicate that the class should not have a table mapped:

    from sqlalchemy.ext.declarative import declared_attr

    class Tablename:
        @declared_attr
        def __tablename__(cls):
            return cls.__name__.lower()

    class Person(Tablename, Base):
        id = Column(Integer, primary_key=True)
        discriminator = Column('type', String(50))
        __mapper_args__ = {'polymorphic_on': discriminator}

    class Engineer(Person):
        __tablename__ = None
        __mapper_args__ = {'polymorphic_identity': 'engineer'}
        primary_language = Column(String(50))

Alternatively, you can make the mixin intelligent enough to only return a `__tablename__` in the event that no table is already mapped in the inheritance hierarchy. To help with this, a `has_inherited_table()` helper function is provided that returns `True` if a parent class already has a mapped table.

As an example, here’s a mixin that will only allow single table inheritance:

    from sqlalchemy.ext.declarative import declared_attr
    from sqlalchemy.ext.declarative import has_inherited_table

    class Tablename(object):
        @declared_attr
        def __tablename__(cls):
            if has_inherited_table(cls):
                return None
            return cls.__name__.lower()

    class Person(Tablename, Base):
        id = Column(Integer, primary_key=True)
        discriminator = Column('type', String(50))
        __mapper_args__ = {'polymorphic_on': discriminator}

    class Engineer(Person):
        primary_language = Column(String(50))
        __mapper_args__ = {'polymorphic_identity': 'engineer'}

If you want to use a similar pattern with a mix of single and joined table inheritance, you would need a slightly different mixin and use it on any joined table child classes in addition to their parent classes:

    from sqlalchemy.ext.declarative import declared_attr
    from sqlalchemy.ext.declarative import has_inherited_table

    class Tablename(object):
        @declared_attr
        def __tablename__(cls):
            if (has_inherited_table(cls) and
                Tablename not in cls.__bases__):
                return None
            return cls.__name__.lower()

    class Person(Tablename, Base):
        id = Column(Integer, primary_key=True)
        discriminator = Column('type', String(50))
        __mapper_args__ = {'polymorphic_on': discriminator}

    # This is single table inheritance
    class Engineer(Person):
        primary_language = Column(String(50))
        __mapper_args__ = {'polymorphic_identity': 'engineer'}

    # This is joined table inheritance
    class Manager(Tablename, Person):
        id = Column(Integer, ForeignKey('person.id'), primary_key=True)
        preferred_recreation = Column(String(50))
        __mapper_args__ = {'polymorphic_identity': 'engineer'}

### Combining Table/Mapper Arguments from Multiple Mixins

In the case of `__table_args__` or `__mapper_args__` specified with declarative mixins, you may want to combine some parameters from several mixins with those you wish to define on the class iteself. The `declared_attr` decorator can be used here to create user-defined collation routines that pull from multiple collections:

    from sqlalchemy.ext.declarative import declared_attr

    class MySQLSettings(object):
        __table_args__ = {'mysql_engine':'InnoDB'}

    class MyOtherMixin(object):
        __table_args__ = {'info':'foo'}

    class MyModel(MySQLSettings, MyOtherMixin, Base):
        __tablename__='my_model'

        @declared_attr
        def __table_args__(cls):
            args = dict()
            args.update(MySQLSettings.__table_args__)
            args.update(MyOtherMixin.__table_args__)
            return args

        id =  Column(Integer, primary_key=True)

### Creating Indexes with Mixins

To define a named, potentially multicolumn `Index` that applies to all tables derived from a mixin, use the “inline” form of `Index` and establish it as part of `__table_args__`:

    class MyMixin(object):
        a =  Column(Integer)
        b =  Column(Integer)

        @declared_attr
        def __table_args__(cls):
            return (Index('test_idx_%s' % cls.__tablename__, 'a', 'b'),)

    class MyModel(MyMixin, Base):
        __tablename__ = 'atable'
        c =  Column(Integer,primary_key=True)

## Special Directives

### `__declare_last__()`

The `__declare_last__()` hook allows definition of a class level function that is automatically called by the `MapperEvents.after_configured()` event, which occurs after mappings are assumed to be completed and the ‘configure’ step has finished:

    class MyClass(Base):
        @classmethod
        def __declare_last__(cls):
            ""
            # do something with mappings

New in version 0.7.3.

### `__abstract__`

`__abstract__` causes declarative to skip the production of a table or mapper for the class entirely. A class can be added within a hierarchy in the same way as mixin (see Mixin and Custom Base Classes), allowing subclasses to extend just from the special class:

    class SomeAbstractBase(Base):
        __abstract__ = True

        def some_helpful_method(self):
            ""

        @declared_attr
        def __mapper_args__(cls):
            return {"helpful mapper arguments":True}

    class MyMappedClass(SomeAbstractBase):
        ""

One possible use of `__abstract__` is to use a distinct `MetaData` for different bases:

    Base = declarative_base()

    class DefaultBase(Base):
        __abstract__ = True
        metadata = MetaData()

    class OtherBase(Base):
        __abstract__ = True
        metadata = MetaData()

Above, classes which inherit from `DefaultBase` will use one `MetaData` as the registry of tables, and those which inherit from `OtherBase` will use a different one. The tables themselves can then be created perhaps within distinct databases:

    DefaultBase.metadata.create_all(some_engine)
    OtherBase.metadata_create_all(some_other_engine)

New in version 0.7.3.

## Class Constructor

As a convenience feature, the `declarative_base()` sets a default constructor on classes which takes keyword arguments, and assigns them to the named attributes:

    e = Engineer(primary_language='python')

## Sessions

Note that `declarative` does nothing special with sessions, and is only intended as an easier way to configure mappers and `Table` objects. A typical application setup using `scoped_session` might look like:

    engine = create_engine('postgresql://scott:tiger@localhost/test')
    Session = scoped_session(sessionmaker(autocommit=False,
                                          autoflush=False,
                                          bind=engine))
    Base = declarative_base()

Mapped instances then make usage of `Session` in the usual way.

## API Reference

`sqlalchemy.ext.declarative.``declarative_base`(bind=None, metadata=None, mapper=None, cls=<type 'object'>, name='Base', constructor=<function __init__ at 0x10aadd938>, class_registry=None, metaclass=<class 'sqlalchemy.ext.declarative.api.DeclarativeMeta'>)

  Construct a base class for declarative class definitions.

  The new base class will be given a metaclass that produces appropriate `Table` objects and makes the appropriate `mapper()` calls based on the information provided declaratively in the class and any subclasses of the class.

  Parameters:

  - bind – An optional `Connectable`, will be assigned the `bind` attribute on the `MetaData` instance.
  - metadata – An optional `MetaData` instance. All `Table` objects implicitly declared by subclasses of the base will share this MetaData. A MetaData instance will be created if none is provided. The `MetaData` instance will be available via the metadata attribute of the generated declarative base class.
  - mapper – An optional callable, defaults to `mapper()`. Will be used to map subclasses to their Tables.
  - cls – Defaults to `object`. A type to use as the base for the generated declarative base class. May be a class or tuple of classes.
  - name – Defaults to `Base`. The display name for the generated class. Customizing this is not required, but can improve clarity in tracebacks and debugging.
  - constructor – Defaults to `_declarative_constructor()`, an __init__ implementation that assigns **kwargs for declared fields and relationships to an instance. If `None` is supplied, no __init__ will be provided and construction will fall back to cls.__init__ by way of the normal Python semantics.
  - class_registry – optional dictionary that will serve as the registry of class names-> mapped classes when string names are used to identify classes inside of `relationship()` and others. Allows two or more declarative base classes to share the same registry of class names for simplified inter-base relationships.
  - metaclass – Defaults to `DeclarativeMeta`. A metaclass or __metaclass__ compatible callable to use as the meta type of the generated declarative base class.

class `sqlalchemy.ext.declarative.``declared_attr`(fget, *arg, **kw)

  Mark a class-level method as representing the definition of a mapped property or special declarative member name.

  @declared_attr turns the attribute into a scalar-like property that can be invoked from the uninstantiated class. Declarative treats attributes specifically marked with @declared_attr as returning a construct that is specific to mapping or declarative table configuration. The name of the attribute is that of what the non-dynamic version of the attribute would be.

  @declared_attr is more often than not applicable to mixins, to define relationships that are to be applied to different implementors of the class:

      class ProvidesUser(object):
          "A mixin that adds a 'user' relationship to classes."

          @declared_attr
          def user(self):
              return relationship("User")

  It also can be applied to mapped classes, such as to provide a “polymorphic” scheme for inheritance:

      class Employee(Base):
          id = Column(Integer, primary_key=True)
          type = Column(String(50), nullable=False)

          @declared_attr
          def __tablename__(cls):
              return cls.__name__.lower()

          @declared_attr
          def __mapper_args__(cls):
              if cls.__name__ == 'Employee':
                  return {
                          "polymorphic_on":cls.type,
                          "polymorphic_identity":"Employee"
                  }
              else:
                  return {"polymorphic_identity":cls.__name__}

  Changed in version 0.8: `declared_attr` can be used with non-ORM or extension attributes, such as user-defined attributes or `association_proxy()` objects, which will be assigned to the class at class construction time.

`sqlalchemy.ext.declarative.api.``_declarative_constructor`(self, **kwargs)

  A simple constructor that allows initialization from kwargs.

  Sets attributes on the constructed instance using the names and values in `kwargs`.

  Only keys that are present as attributes of the instance’s class are allowed. These could be, for example, any mapped columns or relationships.

`sqlalchemy.ext.declarative.``has_inherited_table`(cls)

  Given a class, return True if any of the classes it inherits from has a mapped table, otherwise return False.

`sqlalchemy.ext.declarative.``synonym_for`(name, map_column=False)

  Decorator, make a Python @property a query synonym for a column.

  A decorator version of `synonym()`. The function being decorated is the ‘descriptor’, otherwise passes its arguments through to synonym():

      @synonym_for('col')
      @property
      def prop(self):
          return 'special sauce'

  The regular `synonym()` is also usable directly in a declarative setting and may be convenient for read/write properties:

      prop = synonym('col', descriptor=property(_read_prop, _write_prop))

`sqlalchemy.ext.declarative.``comparable_using`(comparator_factory)

  Decorator, allow a Python @property to be used in query criteria.

  This is a decorator front end to `comparable_property()` that passes through the comparator_factory and the function being decorated:

      @comparable_using(MyComparatorType)
      @property
      def prop(self):
          return 'special sauce'

  The regular `comparable_property()` is also usable directly in a declarative setting and may be convenient for read/write properties:

      prop = comparable_property(MyComparatorType)

`sqlalchemy.ext.declarative.``instrument_declarative`(cls, registry, metadata)

  Given a class, configure the class declaratively, using the given registry, which can be any dictionary, and MetaData object.

class `sqlalchemy.ext.declarative.``AbstractConcreteBase`

  A helper class for ‘concrete’ declarative mappings.

  `AbstractConcreteBase` will use the `polymorphic_union()` function automatically, against all tables mapped as a subclass to this class. The function is called via the `__declare_last__()` function, which is essentially a hook for the `MapperEvents.after_configured()` event.

  `AbstractConcreteBase` does not produce a mapped table for the class itself. Compare to `ConcreteBase`, which does.

  Example:

      from sqlalchemy.ext.declarative import AbstractConcreteBase

      class Employee(AbstractConcreteBase, Base):
          pass

      class Manager(Employee):
          __tablename__ = 'manager'
          employee_id = Column(Integer, primary_key=True)
          name = Column(String(50))
          manager_data = Column(String(40))
          __mapper_args__ = {
                          'polymorphic_identity':'manager',
                          'concrete':True}

class `sqlalchemy.ext.declarative.``ConcreteBase`

  A helper class for ‘concrete’ declarative mappings.

  `ConcreteBase` will use the `polymorphic_union()` function automatically, against all tables mapped as a subclass to this class. The function is called via the `__declare_last__()` function, which is essentially a hook for the `MapperEvents.after_configured()` event.

  `ConcreteBase` produces a mapped table for the class itself. Compare to `AbstractConcreteBase`, which does not.

  Example:

      from sqlalchemy.ext.declarative import ConcreteBase

      class Employee(ConcreteBase, Base):
          __tablename__ = 'employee'
          employee_id = Column(Integer, primary_key=True)
          name = Column(String(50))
          __mapper_args__ = {
                          'polymorphic_identity':'employee',
                          'concrete':True}

      class Manager(Employee):
          __tablename__ = 'manager'
          employee_id = Column(Integer, primary_key=True)
          name = Column(String(50))
          manager_data = Column(String(40))
          __mapper_args__ = {
                          'polymorphic_identity':'manager',
                          'concrete':True}

class `sqlalchemy.ext.declarative.``DeferredReflection`

  A helper class for construction of mappings based on a deferred reflection step.

  Normally, declarative can be used with reflection by setting a `Table` object using autoload=True as the `__table__` attribute on a declarative class. The caveat is that the `Table` must be fully reflected, or at the very least have a primary key column, at the point at which a normal declarative mapping is constructed, meaning the `Engine` must be available at class declaration time.

  The `DeferredReflection` mixin moves the construction of mappers to be at a later point, after a specific method is called which first reflects all `Table` objects created so far. Classes can define it as such:

      from sqlalchemy.ext.declarative import declarative_base
      from sqlalchemy.ext.declarative import DeferredReflection
      Base = declarative_base()

      class MyClass(DeferredReflection, Base):
          __tablename__ = 'mytable'

  Above, `MyClass` is not yet mapped. After a series of classes have been defined in the above fashion, all tables can be reflected and mappings created using `DeferredReflection.prepare()`:

      engine = create_engine("someengine://...")
      DeferredReflection.prepare(engine)

  The `DeferredReflection` mixin can be applied to individual classes, used as the base for the declarative base itself, or used in a custom abstract class. Using an abstract base allows that only a subset of classes to be prepared for a particular prepare step, which is necessary for applications that use more than one engine. For example, if an application has two engines, you might use two bases, and prepare each separately, e.g.:

      class ReflectedOne(DeferredReflection, Base):
          __abstract__ = True

      class ReflectedTwo(DeferredReflection, Base):
          __abstract__ = True

      class MyClass(ReflectedOne):
          __tablename__ = 'mytable'

      class MyOtherClass(ReflectedOne):
          __tablename__ = 'myothertable'

      class YetAnotherClass(ReflectedTwo):
          __tablename__ = 'yetanothertable'

      # ... etc.

  Above, the class hierarchies for `ReflectedOne` and `ReflectedTwo` can be configured separately:

      ReflectedOne.prepare(engine_one)
      ReflectedTwo.prepare(engine_two)

  New in version 0.8.