.. _orm_declarative_styles_toplevel:

==========================
Declarative Mapping Styles
==========================

As introduced at :ref:`orm_declarative_mapping`, the **Declarative Mapping** is
the typical way that mappings are constructed in modern SQLAlchemy.   This
section will provide an overview of forms that may be used for Declarative
mapper configuration.


.. _orm_explicit_declarative_base:

.. _orm_declarative_generated_base_class:

Using a Declarative Base Class
-------------------------------

The most common approach is to generate a "Declarative Base" class by
subclassing the :class:`_orm.DeclarativeBase` superclass::

    from sqlalchemy.orm import DeclarativeBase


    # declarative base class
    class Base(DeclarativeBase):
        pass

The Declarative Base class may also be created given an existing
:class:`_orm.registry` by assigning it as a class variable named
``registry``::

    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import registry

    reg = registry()


    # declarative base class
    class Base(DeclarativeBase):
        registry = reg

.. versionchanged:: 2.0 The :class:`_orm.DeclarativeBase` superclass supersedes
   the use of the :func:`_orm.declarative_base` function and
   :meth:`_orm.registry.generate_base` methods; the superclass approach
   integrates with :pep:`484` tools without the use of plugins.
   See :ref:`whatsnew_20_orm_declarative_typing` for migration notes.

With the declarative base class, new mapped classes are declared as subclasses
of the base::

    from datetime import datetime
    from typing import Optional

    from sqlalchemy import ForeignKey
    from sqlalchemy import func
    from sqlalchemy import Integer
    from sqlalchemy import String
    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import Mapped
    from sqlalchemy.orm import mapped_column
    from sqlalchemy.orm import relationship


    class Base(DeclarativeBase):
        pass


    class User(Base):
        __tablename__ = "user"

        id = mapped_column(Integer, primary_key=True)
        name: Mapped[str]
        fullname: Mapped[Optional[str]]
        nickname: Mapped[Optional[str]] = mapped_column(String(64))
        create_date: Mapped[datetime] = mapped_column(insert_default=func.now())

        addresses: Mapped[List["Address"]] = relationship(back_populates="user")


    class Address(Base):
        __tablename__ = "address"

        id = mapped_column(Integer, primary_key=True)
        user_id = mapped_column(ForeignKey("user.id"))
        email_address: Mapped[str]

        user: Mapped["User"] = relationship(back_populates="addresses")

Above, the ``Base`` class serves as a base for new classes that are to be
mapped, as above new mapped classes ``User`` and ``Address`` are constructed.

For each subclass constructed, the body of the class then follows the
declarative mapping approach which defines both a :class:`_schema.Table` as
well as a :class:`_orm.Mapper` object behind the scenes which comprise a full
mapping.

.. seealso::

    :ref:`orm_declarative_table_config_toplevel` - describes how to specify
    the components of the mapped :class:`_schema.Table` to be generated,
    including notes and options on the use of the :func:`_orm.mapped_column`
    construct and how it interacts with the :class:`_orm.Mapped` annotation
    type

    :ref:`orm_declarative_mapper_config_toplevel` - describes all other
    aspects of ORM mapper configuration within Declarative including
    :func:`_orm.relationship` configuration, SQL expressions and
    :class:`_orm.Mapper` parameters


.. _orm_declarative_decorator:

Declarative Mapping using a Decorator (no declarative base)
------------------------------------------------------------

As an alternative to using the "declarative base" class is to apply
declarative mapping to a class explicitly, using either an imperative technique
similar to that of a "classical" mapping, or more succinctly by using
a decorator.  The :meth:`_orm.registry.mapped` function is a class decorator
that can be applied to any Python class with no hierarchy in place.  The
Python class otherwise is configured in declarative style normally.

The example below sets up the identical mapping as seen in the
previous section, using the :meth:`_orm.registry.mapped`
decorator rather than using the :class:`_orm.DeclarativeBase` superclass::

    from datetime import datetime
    from typing import List
    from typing import Optional

    from sqlalchemy import ForeignKey
    from sqlalchemy import func
    from sqlalchemy import Integer
    from sqlalchemy import String
    from sqlalchemy.orm import Mapped
    from sqlalchemy.orm import mapped_column
    from sqlalchemy.orm import registry
    from sqlalchemy.orm import relationship

    mapper_registry = registry()


    @mapper_registry.mapped
    class User:
        __tablename__ = "user"

        id = mapped_column(Integer, primary_key=True)
        name: Mapped[str]
        fullname: Mapped[Optional[str]]
        nickname: Mapped[Optional[str]] = mapped_column(String(64))
        create_date: Mapped[datetime] = mapped_column(insert_default=func.now())

        addresses: Mapped[List["Address"]] = relationship(back_populates="user")


    @mapper_registry.mapped
    class Address:
        __tablename__ = "address"

        id = mapped_column(Integer, primary_key=True)
        user_id = mapped_column(ForeignKey("user.id"))
        email_address: Mapped[str]

        user: Mapped["User"] = relationship(back_populates="addresses")

When using the above style, the mapping of a particular class will **only**
proceed if the decorator is applied to that class directly. For inheritance
mappings (described in detail at :ref:`inheritance_toplevel`), the decorator
should be applied to each subclass that is to be mapped::

    from sqlalchemy.orm import registry

    mapper_registry = registry()


    @mapper_registry.mapped
    class Person:
        __tablename__ = "person"

        person_id = mapped_column(Integer, primary_key=True)
        type = mapped_column(String, nullable=False)

        __mapper_args__ = {
            "polymorphic_on": type,
            "polymorphic_identity": "person",
        }


    @mapper_registry.mapped
    class Employee(Person):
        __tablename__ = "employee"

        person_id = mapped_column(ForeignKey("person.person_id"), primary_key=True)

        __mapper_args__ = {
            "polymorphic_identity": "employee",
        }

Both the :ref:`declarative table <orm_declarative_table>` and
:ref:`imperative table <orm_imperative_table_configuration>`
table configuration styles may be used with either the Declarative Base
or decorator styles of Declarative mapping.

The decorator form of mapping is useful when combining a
SQLAlchemy declarative mapping with other class instrumentation systems
such as dataclasses_ and attrs_, though note that SQLAlchemy 2.0 now features
dataclasses integration with Declarative Base classes as well.


.. _dataclass: https://docs.python.org/3/library/dataclasses.html
.. _dataclasses: https://docs.python.org/3/library/dataclasses.html
.. _attrs: https://pypi.org/project/attrs/
