Release: 2.0.44 | Release Date: October 10, 2025

# SQLAlchemy 2.0 Documentation

# Declarative Extensions

Extensions specific to the Declarative mapping API.

Changed in version 1.4: The vast majority of the Declarative extension is now integrated into the SQLAlchemy ORM and is importable from the `sqlalchemy.orm` namespace. See the documentation at Declarative Mapping for new documentation. For an overview of the change, see Declarative is now integrated into the ORM with new features.

Object Name | Description
--- | ---
AbstractConcreteBase |
ConcreteBase | A helper class for ‘concrete’ declarative mappings.
DeferredReflection | A helper class for construction of mappings based on a deferred reflection step.

class sqlalchemy.ext.declarative.AbstractConcreteBase

  inherits from `sqlalchemy.ext.declarative.extensions.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_first__()` function, which is essentially a hook for the `before_configured()` event.

  `AbstractConcreteBase` applies `Mapper` for its immediately inheriting class, as would occur for any other declarative mapped class. However, the `Mapper` is not mapped to any particular `Table` object. Instead, it’s mapped directly to the “polymorphic” selectable produced by `polymorphic_union()`, and performs no persistence operations on its own. Compare to `ConcreteBase`, which maps its immediately inheriting class to an actual `Table` that stores rows directly.

  Note

  The `AbstractConcreteBase` delays the mapper creation of the base class until all the subclasses have been defined, as it needs to create a mapping against a selectable that will include all subclass tables. In order to achieve this, it waits for the mapper configuration event to occur, at which point it scans through all the configured subclasses and sets up a mapping that will query against all subclasses at once.

  While this event is normally invoked automatically, in the case of `AbstractConcreteBase`, it may be necessary to invoke it explicitly after all subclass mappings are defined, if the first operation is to be a query against this base class. To do so, once all the desired classes have been configured, the `registry.configure()` method on the `registry` in use can be invoked, which is available in relation to a particular declarative base class:

      Base.registry.configure()

  Example:

      from sqlalchemy.orm import DeclarativeBase
      from sqlalchemy.ext.declarative import AbstractConcreteBase

      class Base(DeclarativeBase):
          pass

      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,
          }

      Base.registry.configure()

  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 API feature.

  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:

      from sqlalchemy.ext.declarative import AbstractConcreteBase

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

      class Employee(AbstractConcreteBase, Base):
          strict_attrs = True

          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,
          }

      Base.registry.configure()

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

      session.execute(select(Employee).filter(Employee.company.has(id=5)))

  Parameters:

    strict_attrs –

    when specified on the base class, “strict” attribute mode is enabled which attempts to limit ORM mapped attributes on the base class to only those that are immediately present, while still preserving “polymorphic” loading behavior.

    Added in version 2.0.

  See also

  `ConcreteBase`

  Concrete Table Inheritance

  Abstract Concrete Classes

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,
          }

  The name of the discriminator column used by `polymorphic_union()` defaults to the name `type`. To suit the use case of a mapping where an actual column in a mapped table is already named `type`, the discriminator name can be configured by setting the `_concrete_discriminator_name` attribute:

      class Employee(ConcreteBase, Base):
          _concrete_discriminator_name = "_concrete_discriminator"

  Added in version 1.3.19: Added the `_concrete_discriminator_name` attribute to `ConcreteBase` so that the virtual discriminator column name can be customized.

  Changed in version 1.4.2: The `_concrete_discriminator_name` attribute need only be placed on the basemost class to take correct effect for all subclasses. An explicit error message is now raised if the mapped column names conflict with the discriminator name, whereas in the 1.3.x series there would be some warnings and then a non-useful query would be generated.

  See also

  `AbstractConcreteBase`

  Concrete Table Inheritance

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_with=engine 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)

  See also

  Using DeferredReflection - in the Table Configuration with Declarative section.

  Member Name | Description
  --- | ---
  prepare() | Reflect all `Table` objects for all current `DeferredReflection` subclasses

  classmethod `sqlalchemy.ext.declarative.DeferredReflection.`prepare(bind: Engine | Connection, **reflect_kw: Any) → None

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

    Parameters:

      - bind –

        `Engine` or `Connection` instance

        ..versionchanged:: 2.0.16 a `Connection` is also accepted.

      - **reflect_kw –

        additional keyword arguments passed to `MetaData.reflect()`, such as `MetaData.reflect.views`.

        Added in version 2.0.16.