Release: 1.2.19 | Release Date: April 15, 2019

# SQLAlchemy 1.2 Documentation

# Declarative API

## API Reference

`sqlalchemy.ext.declarative.``declarative_base`(bind=None, metadata=None, mapper=None, cls=<class 'object'>, name='Base', constructor=<function _declarative_constructor>, 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.

  Changed in version 1.1: if `declarative_base.cls` is a single class (rather than a tuple), the constructed base class will inherit its docstring.

  See also

  `as_declarative()`

`sqlalchemy.ext.declarative.``as_declarative`(**kw)

  Class decorator for `declarative_base()`.

  Provides a syntactical shortcut to the `cls` argument sent to `declarative_base()`, allowing the base class to be converted in-place to a “declarative” base:

      from sqlalchemy.ext.declarative import as_declarative

      @as_declarative()
      class Base(object):
          @declared_attr
          def __tablename__(cls):
              return cls.__name__.lower()
          id = Column(Integer, primary_key=True)

      class MyMappedClass(Base):
          # ...

  All keyword arguments passed to `as_declarative()` are passed along to `declarative_base()`.

  See also

  `declarative_base()`

class `sqlalchemy.ext.declarative.``declared_attr`(fget, cascading=False)

  Bases: `sqlalchemy.orm.base._MappedAttribute`, `builtins.property`

  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__}

  `cascading`

    Mark a `declared_attr` as cascading.

    This is a special-use modifier which indicates that a column or MapperProperty-based declared attribute should be configured distinctly per mapped subclass, within a mapped-inheritance scenario.

    Warning

    The `declared_attr.cascading` modifier has several limitations:

    - The flag only applies to the use of `declared_attr` on declarative mixin classes and `__abstract__` classes; it currently has no effect when used on a mapped class directly.

    - The flag only applies to normally-named attributes, e.g. not any special underscore attributes such as `__tablename__`. On these attributes it has no effect.

    - The flag currently does not allow further overrides down the class hierarchy; if a subclass tries to override the attribute, a warning is emitted and the overridden attribute is skipped. This is a limitation that it is hoped will be resolved at some point.

    Below, both MyClass as well as MySubClass will have a distinct `id` Column object established:

        class HasIdMixin(object):
            @declared_attr.cascading
            def id(cls):
                if has_inherited_table(cls):
                    return Column(
                        ForeignKey('myclass.id'), primary_key=True)
                else:
                    return Column(Integer, primary_key=True)

        class MyClass(HasIdMixin, Base):
            __tablename__ = 'myclass'
            # ...

        class MySubClass(MyClass):
            ""
            # ...

    The behavior of the above configuration is that `MySubClass` will refer to both its own `id` column as well as that of `MyClass` underneath the attribute named `some_id`.

    See also

    Inheritance Configuration

    Mixing in Columns in Inheritance Scenarios

`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.

  This is used in declarative mixins to build attributes that behave differently for the base class vs. a subclass in an inheritance hierarchy.

  See also

  Controlling table inheritance with mixins

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

  Decorator that produces an `orm.synonym()` attribute in conjunction with a Python descriptor.

  The function being decorated is passed to `orm.synonym()` as the `orm.synonym.descriptor` parameter:

      class MyClass(Base):
          __tablename__ = 'my_table'

          id = Column(Integer, primary_key=True)
          _job_status = Column("job_status", String(50))

          @synonym_for("job_status")
          @property
          def job_status(self):
              return "Status: %s" % self._job_status

  The hybrid properties feature of SQLAlchemy is typically preferred instead of synonyms, which is a more legacy feature.

  See also

  Synonyms - Overview of synonyms

  `orm.synonym()` - the mapper-level function

  Using Descriptors and Hybrids - The Hybrid Attribute extension provides an updated approach to augmenting attribute behavior more flexibly than can be achieved with synonyms.

`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`

  Bases: `sqlalchemy.ext.declarative.api.ConcreteBase`

  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 `after_configured()` event.

  `AbstractConcreteBase` does produce a mapped class for the base class, however it is not persisted to any table; it is instead mapped directly to the “polymorphic” selectable directly and is only used for selecting. Compare to `ConcreteBase`, which does create a persisted table for the base class.

  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}

  The abstract base class is handled by declarative in a special way; at class configuration time, it behaves like a declarative mixin or an `__abstract__` base class. Once classes are configured and mappings are produced, it then gets mapped itself, but after all of its descendants. This is a very unique system of mapping not found in any other SQLAlchemy system.

  Using this approach, we can specify columns and properties that will take place on mapped subclasses, in the way that we normally do as in Mixin and Custom Base Classes:

      class Company(Base):
          __tablename__ = 'company'
          id = Column(Integer, primary_key=True)

      class Employee(AbstractConcreteBase, Base):
          employee_id = Column(Integer, primary_key=True)

          @declared_attr
          def company_id(cls):
              return Column(ForeignKey('company.id'))

          @declared_attr
          def company(cls):
              return relationship("Company")

      class Manager(Employee):
          __tablename__ = 'manager'

          name = Column(String(50))
          manager_data = Column(String(40))

          __mapper_args__ = {
              'polymorphic_identity':'manager',
              'concrete':True}

  When we make use of our mappings however, both `Manager` and `Employee` will have an independently usable `.company` attribute:

      session.query(Employee).filter(Employee.company.has(id=5))

  Changed in version 1.0.0: - The mechanics of `AbstractConcreteBase` have been reworked to support relationships established directly on the abstract base, without any special configurational steps.

  See also

  `ConcreteBase`

  Concrete Table Inheritance

  inheritance_concrete_helpers

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 `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}

  See also

  `AbstractConcreteBase`

  Concrete Table Inheritance

  inheritance_concrete_helpers

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 `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)

  classmethod `prepare`(engine)

    Reflect all `Table` objects for all current `DeferredReflection` subclasses

### 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

#### `__declare_first__()`

Like `__declare_last__()`, but is called at the beginning of mapper configuration via the `MapperEvents.before_configured()` event:

    class MyClass(Base):
        @classmethod
        def __declare_first__(cls):
            ""
            # do something before mappings are configured

New in version 0.9.3.

#### `__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)

#### `__table_cls__`

Allows the callable / class used to generate a `Table` to be customized. This is a very open-ended hook that can allow special customizations to a `Table` that one generates here:

    class MyMixin(object):
        @classmethod
        def __table_cls__(cls, name, metadata, *arg, **kw):
            return Table(
                "my_" + name,
                metadata, *arg, **kw
            )

The above mixin would cause all `Table` objects generated to include the prefix `"my_"`, followed by the name normally specified using the `__tablename__` attribute.

`__table_cls__` also supports the case of returning `None`, which causes the class to be considered as single-table inheritance vs. its subclass. This may be useful in some customization schemes to determine that single-table inheritance should take place based on the arguments for the table itself, such as, define as single-inheritance if there is no primary key present:

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

        @classmethod
        def __table_cls__(cls, *arg, **kw):
            for obj in arg[1:]:
                if (isinstance(obj, Column) and obj.primary_key) or \
                        isinstance(obj, PrimaryKeyConstraint):
                    return Table(*arg, **kw)

            return None

    class Person(AutoTable, Base):
        id = Column(Integer, primary_key=True)

    class Employee(Person):
        employee_name = Column(String)

The above `Employee` class would be mapped as single-table inheritance against `Person`; the `employee_name` column would be added as a member of the `Person` table.

New in version 1.0.0.