
.. _orm_declarative_table_config_toplevel:

=============================================
Table Configuration with Declarative
=============================================

As introduced at :ref:`orm_declarative_mapping`, the Declarative style
includes the ability to generate a mapped :class:`_schema.Table` object
at the same time, or to accommodate a :class:`_schema.Table` or other
:class:`_sql.FromClause` object directly.

The following examples assume a declarative base class as::

    from sqlalchemy.orm import DeclarativeBase


    class Base(DeclarativeBase):
        pass

All of the examples that follow illustrate a class inheriting from the above
``Base``.  The decorator style introduced at :ref:`orm_declarative_decorator`
is fully supported with all the following examples as well, as are legacy
forms of Declarative Base including base classes generated by
:func:`_orm.declarative_base`.


.. _orm_declarative_table:

Declarative Table with ``mapped_column()``
------------------------------------------

When using Declarative, the body of the class to be mapped in most cases
includes an attribute ``__tablename__`` that indicates the string name of a
:class:`_schema.Table` that should be generated along with the mapping. The
:func:`_orm.mapped_column` construct, which features additional ORM-specific
configuration capabilities not present in the plain :class:`_schema.Column`
class, is then used within the class body to indicate columns in the table. The
example below illustrates the most basic use of this construct within a
Declarative mapping::


    from sqlalchemy import Integer, String
    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import mapped_column


    class Base(DeclarativeBase):
        pass


    class User(Base):
        __tablename__ = "user"

        id = mapped_column(Integer, primary_key=True)
        name = mapped_column(String(50), nullable=False)
        fullname = mapped_column(String)
        nickname = mapped_column(String(30))

Above, :func:`_orm.mapped_column` constructs are placed inline within the class
definition as class level attributes. At the point at which the class is
declared, the Declarative mapping process will generate a new
:class:`_schema.Table` object against the :class:`_schema.MetaData` collection
associated with the Declarative ``Base``; each instance of
:func:`_orm.mapped_column` will then be used to generate a
:class:`_schema.Column` object during this process, which will become part of
the :attr:`.schema.Table.columns` collection of this :class:`_schema.Table`
object.

In the above example, Declarative will build a :class:`_schema.Table`
construct that is equivalent to the following::

    # equivalent Table object produced
    user_table = Table(
        "user",
        Base.metadata,
        Column("id", Integer, primary_key=True),
        Column("name", String(50)),
        Column("fullname", String()),
        Column("nickname", String(30)),
    )

When the ``User`` class above is mapped, this :class:`_schema.Table` object
can be accessed directly via the ``__table__`` attribute; this is described
further at :ref:`orm_declarative_metadata`.

.. sidebar::  ``mapped_column()`` supersedes the use of ``Column()``

  Users of 1.x SQLAlchemy will note the use of the :func:`_orm.mapped_column`
  construct, which is new as of the SQLAlchemy 2.0 series.  This ORM-specific
  construct is intended first and foremost to be a drop-in replacement for
  the use of :class:`_schema.Column` within Declarative mappings only, adding
  new ORM-specific convenience features such as the ability to establish
  :paramref:`_orm.mapped_column.deferred` within the construct, and most
  importantly to indicate to typing tools such as Mypy_ and Pylance_ an
  accurate representation of how the attribute will behave at runtime at
  both the class level as well as the instance level.  As will be seen in
  the following sections, it's also at the forefront of a new
  annotation-driven configuration style introduced in SQLAlchemy 2.0.

  Users of legacy code should be aware that the :class:`_schema.Column` form
  will always work in Declarative in the same way it always has. The different
  forms of attribute mapping may also be mixed within a single mapping on an
  attribute by attribute basis, so migration to the new form can be at
  any pace.   See the section :ref:`whatsnew_20_orm_declarative_typing` for
  a step by step guide to migrating a Declarative model to the new form.


The :func:`_orm.mapped_column` construct accepts all arguments that are
accepted by the :class:`_schema.Column` construct, as well as additional
ORM-specific arguments. The :paramref:`_orm.mapped_column.__name` field,
indicating the name of the database column, is typically omitted, as the
Declarative process will make use of the attribute name given to the construct
and assign this as the name of the column (in the above example, this refers to
the names ``id``, ``name``, ``fullname``, ``nickname``). Assigning an alternate
:paramref:`_orm.mapped_column.__name` is valid as well, where the resulting
:class:`_schema.Column` will use the given name in SQL and DDL statements,
while the ``User`` mapped class will continue to allow access to the attribute
using the attribute name given, independent of the name given to the column
itself (more on this at :ref:`mapper_column_distinct_names`).

.. tip::

    The :func:`_orm.mapped_column` construct is **only valid within a
    Declarative class mapping**.   When constructing a :class:`_schema.Table`
    object using Core as well as when using
    :ref:`imperative table <orm_imperative_table_configuration>` configuration,
    the :class:`_schema.Column` construct is still required in order to
    indicate the presence of a database column.

.. seealso::

    :ref:`mapping_columns_toplevel` - contains additional notes on affecting
    how :class:`_orm.Mapper` interprets incoming :class:`.Column` objects.

.. _orm_declarative_mapped_column:

Using Annotated Declarative Table (Type Annotated Forms for ``mapped_column()``)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The :func:`_orm.mapped_column` construct is capable of deriving its column-configuration
information from :pep:`484` type annotations associated with the attribute
as declared in the Declarative mapped class.   These type annotations,
if used,  **must**
be present within a special SQLAlchemy type called :class:`_orm.Mapped`, which
is a generic_ type that then indicates a specific Python type within it.

Below illustrates the mapping from the previous section, adding the use of
:class:`_orm.Mapped`::

    from typing import Optional

    from sqlalchemy import String
    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import Mapped
    from sqlalchemy.orm import mapped_column


    class Base(DeclarativeBase):
        pass


    class User(Base):
        __tablename__ = "user"

        id: Mapped[int] = mapped_column(primary_key=True)
        name: Mapped[str] = mapped_column(String(50))
        fullname: Mapped[Optional[str]]
        nickname: Mapped[Optional[str]] = mapped_column(String(30))

Above, when Declarative processes each class attribute, each
:func:`_orm.mapped_column` will derive additional arguments from the
corresponding :class:`_orm.Mapped` type annotation on the left side, if
present.   Additionally, Declarative will generate an empty
:func:`_orm.mapped_column` directive implicitly, whenever a
:class:`_orm.Mapped` type annotation is encountered that does not have
a value assigned to the attribute (this form is inspired by the similar
style used in Python dataclasses_); this :func:`_orm.mapped_column` construct
proceeds to derive its configuration from the :class:`_orm.Mapped`
annotation present.

.. _orm_declarative_mapped_column_nullability:

``mapped_column()`` derives the datatype and nullability from the ``Mapped`` annotation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The two qualities that :func:`_orm.mapped_column` derives from the
:class:`_orm.Mapped` annotation are:

* **datatype** - the Python type given inside :class:`_orm.Mapped`, as contained
  within the ``typing.Optional`` construct if present, is associated with a
  :class:`_sqltypes.TypeEngine` subclass such as :class:`.Integer`, :class:`.String`,
  :class:`.DateTime`, or :class:`.Uuid`, to name a few common types.

  The datatype is determined based on a dictionary of Python type to
  SQLAlchemy datatype.   This dictionary is completely customizable,
  as detailed in the next section :ref:`orm_declarative_mapped_column_type_map`.
  The default type map is implemented as in the code example below::

      from typing import Any
      from typing import Dict
      from typing import Type

      import datetime
      import decimal
      import uuid

      from sqlalchemy import types

      # default type mapping, deriving the type for mapped_column()
      # from a Mapped[] annotation
      type_map: Dict[Type[Any], TypeEngine[Any]] = {
          bool: types.Boolean(),
          bytes: types.LargeBinary(),
          datetime.date: types.Date(),
          datetime.datetime: types.DateTime(),
          datetime.time: types.Time(),
          datetime.timedelta: types.Interval(),
          decimal.Decimal: types.Numeric(),
          float: types.Float(),
          int: types.Integer(),
          str: types.String(),
          uuid.UUID: types.Uuid(),
      }

  If the :func:`_orm.mapped_column` construct indicates an explicit type
  as passed to the :paramref:`_orm.mapped_column.__type` argument, then
  the given Python type is disregarded.

* **nullability** - The :func:`_orm.mapped_column` construct will indicate
  its :class:`_schema.Column` as ``NULL`` or ``NOT NULL`` first and foremost by
  the presence of the :paramref:`_orm.mapped_column.nullable` parameter, passed
  either as ``True`` or ``False``. Additionally , if the
  :paramref:`_orm.mapped_column.primary_key` parameter is present and set to
  ``True``, that will also imply that the column should be ``NOT NULL``.

  In the absence of **both** of these parameters, the presence of
  ``typing.Optional[]`` within the :class:`_orm.Mapped` type annotation will be
  used to determine nullability, where ``typing.Optional[]`` means ``NULL``,
  and the absence of ``typing.Optional[]`` means ``NOT NULL``. If there is no
  ``Mapped[]`` annotation present at all, and there is no
  :paramref:`_orm.mapped_column.nullable` or
  :paramref:`_orm.mapped_column.primary_key` parameter, then SQLAlchemy's usual
  default for :class:`_schema.Column` of ``NULL`` is used.

  In the example below, the ``id`` and ``data`` columns will be ``NOT NULL``,
  and the ``additional_info`` column will be ``NULL``::

      from typing import Optional

      from sqlalchemy.orm import DeclarativeBase
      from sqlalchemy.orm import Mapped
      from sqlalchemy.orm import mapped_column


      class Base(DeclarativeBase):
          pass


      class SomeClass(Base):
          __tablename__ = "some_table"

          # primary_key=True, therefore will be NOT NULL
          id: Mapped[int] = mapped_column(primary_key=True)

          # not Optional[], therefore will be NOT NULL
          data: Mapped[str]

          # Optional[], therefore will be NULL
          additional_info: Mapped[Optional[str]]

  It is also perfectly valid to have a :func:`_orm.mapped_column` whose
  nullability is **different** from what would be implied by the annotation.
  For example, an ORM mapped attribute may be annotated as allowing ``None``
  within Python code that works with the object as it is first being created
  and populated, however the value will ultimately be written to a database
  column that is ``NOT NULL``.   The :paramref:`_orm.mapped_column.nullable`
  parameter, when present, will always take precedence::

      class SomeClass(Base):
          # ...

          # will be String() NOT NULL, but can be None in Python
          data: Mapped[Optional[str]] = mapped_column(nullable=False)

  Similarly, a non-None attribute that's written to a database column that
  for whatever reason needs to be NULL at the schema level,
  :paramref:`_orm.mapped_column.nullable` may be set to ``True``::

      class SomeClass(Base):
          # ...

          # will be String() NULL, but type checker will not expect
          # the attribute to be None
          data: Mapped[str] = mapped_column(nullable=True)

.. _orm_declarative_mapped_column_type_map:

Customizing the Type Map
~~~~~~~~~~~~~~~~~~~~~~~~

The mapping of Python types to SQLAlchemy :class:`_types.TypeEngine` types
described in the previous section defaults to a hardcoded dictionary
present in the ``sqlalchemy.sql.sqltypes`` module.  However, the :class:`_orm.registry`
object that coordinates the Declarative mapping process will first consult
a local, user defined dictionary of types which may be passed
as the :paramref:`_orm.registry.type_annotation_map` parameter when
constructing the :class:`_orm.registry`, which may be associated with
the :class:`_orm.DeclarativeBase` superclass when first used.

As an example, if we wish to make use of the :class:`_sqltypes.BIGINT` datatype for
``int``, the :class:`_sqltypes.TIMESTAMP` datatype with ``timezone=True`` for
``datetime.datetime``, and then only on Microsoft SQL Server we'd like to use
:class:`_sqltypes.NVARCHAR` datatype when Python ``str`` is used,
the registry and Declarative base could be configured as::

    import datetime

    from sqlalchemy import BIGINT, Integer, NVARCHAR, String, TIMESTAMP
    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import Mapped, mapped_column, registry


    class Base(DeclarativeBase):
        type_annotation_map = {
            int: BIGINT,
            datetime.datetime: TIMESTAMP(timezone=True),
            str: String().with_variant(NVARCHAR, "mssql"),
        }


    class SomeClass(Base):
        __tablename__ = "some_table"

        id: Mapped[int] = mapped_column(primary_key=True)
        date: Mapped[datetime.datetime]
        status: Mapped[str]

Below illustrates the CREATE TABLE statement generated for the above mapping,
first on the Microsoft SQL Server backend, illustrating the ``NVARCHAR`` datatype:

.. sourcecode:: pycon+sql

    >>> from sqlalchemy.schema import CreateTable
    >>> from sqlalchemy.dialects import mssql, postgresql
    >>> print(CreateTable(SomeClass.__table__).compile(dialect=mssql.dialect()))
    {printsql}CREATE TABLE some_table (
      id BIGINT NOT NULL IDENTITY,
      date TIMESTAMP NOT NULL,
      status NVARCHAR(max) NOT NULL,
      PRIMARY KEY (id)
    )

Then on the PostgreSQL backend, illustrating ``TIMESTAMP WITH TIME ZONE``:

.. sourcecode:: pycon+sql

    >>> print(CreateTable(SomeClass.__table__).compile(dialect=postgresql.dialect()))
    {printsql}CREATE TABLE some_table (
      id BIGSERIAL NOT NULL,
      date TIMESTAMP WITH TIME ZONE NOT NULL,
      status VARCHAR NOT NULL,
      PRIMARY KEY (id)
    )

By making use of methods such as :meth:`.TypeEngine.with_variant`, we're able
to build up a type map that's customized to what we need for different backends,
while still being able to use succinct annotation-only :func:`_orm.mapped_column`
configurations.  There are two more levels of Python-type configurability
available beyond this, described in the next two sections.

.. _orm_declarative_mapped_column_type_map_pep593:

Mapping Multiple Type Configurations to Python Types
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As individual Python types may be associated with :class:`_types.TypeEngine`
configurations of any variety by using the :paramref:`_orm.registry.type_annotation_map`
parameter, an additional
capability is the ability to associate a single Python type with different
variants of a SQL type based on additional type qualifiers.  One typical
example of this is mapping the Python ``str`` datatype to ``VARCHAR``
SQL types of different lengths.  Another is mapping different varieties of
``decimal.Decimal`` to differently sized ``NUMERIC`` columns.

Python's typing system provides a great way to add additional metadata to a
Python type which is by using the :pep:`593` ``Annotated`` generic type, which
allows additional information to be bundled along with a Python type. The
:func:`_orm.mapped_column` construct will correctly interpret an ``Annotated``
object by identity when resolving it in the
:paramref:`_orm.registry.type_annotation_map`, as in the example below where we
declare two variants of :class:`.String` and :class:`.Numeric`::

    from decimal import Decimal

    from typing_extensions import Annotated

    from sqlalchemy import Numeric
    from sqlalchemy import String
    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import Mapped
    from sqlalchemy.orm import mapped_column
    from sqlalchemy.orm import registry

    str_30 = Annotated[str, 30]
    str_50 = Annotated[str, 50]
    num_12_4 = Annotated[Decimal, 12]
    num_6_2 = Annotated[Decimal, 6]


    class Base(DeclarativeBase):
        registry = registry(
            type_annotation_map={
                str_30: String(30),
                str_50: String(50),
                num_12_4: Numeric(12, 4),
                num_6_2: Numeric(6, 2),
            }
        )

The Python type passed to the ``Annotated`` container, in the above example the
``str`` and ``Decimal`` types, is important only for the benefit of typing
tools; as far as the :func:`_orm.mapped_column` construct is concerned, it will only need
perform a lookup of each type object in the
:paramref:`_orm.registry.type_annotation_map` dictionary without actually
looking inside of the ``Annotated`` object, at least in this particular
context. Similarly, the arguments passed to ``Annotated`` beyond the underlying
Python type itself are also not important, it's only that at least one argument
must be present for the ``Annotated`` construct to be valid. We can then use
these augmented types directly in our mapping where they will be matched to the
more specific type constructions, as in the following example::

    class SomeClass(Base):
        __tablename__ = "some_table"

        short_name: Mapped[str_30] = mapped_column(primary_key=True)
        long_name: Mapped[str_50]
        num_value: Mapped[num_12_4]
        short_num_value: Mapped[num_6_2]

a CREATE TABLE for the above mapping will illustrate the different variants
of ``VARCHAR`` and ``NUMERIC`` we've configured, and looks like:

.. sourcecode:: pycon+sql

    >>> from sqlalchemy.schema import CreateTable
    >>> print(CreateTable(SomeClass.__table__))
    {printsql}CREATE TABLE some_table (
      short_name VARCHAR(30) NOT NULL,
      long_name VARCHAR(50) NOT NULL,
      num_value NUMERIC(12, 4) NOT NULL,
      short_num_value NUMERIC(6, 2) NOT NULL,
      PRIMARY KEY (short_name)
    )

While variety in linking ``Annotated`` types to different SQL types grants
us a wide degree of flexibility, the next section illustrates a second
way in which ``Annotated`` may be used with Declarative that is even
more open ended.

.. _orm_declarative_mapped_column_pep593:

Mapping Whole Column Declarations to Python Types
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The previous section illustrated using :pep:`593` ``Annotated`` type
instances as keys within the :paramref:`_orm.registry.type_annotation_map`
dictionary.  In this form, the :func:`_orm.mapped_column` construct does not
actually look inside the ``Annotated`` object itself, it's instead
used only as a dictionary key.  However, Declarative also has the ability to extract
an entire pre-established :func:`_orm.mapped_column` construct from
an ``Annotated`` object directly.  Using this form, we can define not only
different varieties of SQL datatypes linked to Python types without using
the :paramref:`_orm.registry.type_annotation_map` dictionary, we can also
set up any number of arguments such as nullability, column defaults,
and constraints in a reusable fashion.

A set of ORM models will usually have some kind of primary
key style that is common to all mapped classes.   There also may be
common column configurations such as timestamps with defaults and other fields of
pre-established sizes and configurations.   We can compose these configurations
into :func:`_orm.mapped_column` instances that we then bundle directly into
instances of ``Annotated``, which are then re-used in any number of class
declarations.  Declarative will unpack an ``Annotated`` object
when provided in this manner, skipping over any other directives that don't
apply to SQLAlchemy and searching only for SQLAlchemy ORM constructs.

The example below illustrates a variety of pre-configured field types used
in this way, where we define ``intpk`` that represents an :class:`.Integer` primary
key column, ``timestamp`` that represents a :class:`.DateTime` type
which will use ``CURRENT_TIMESTAMP`` as a DDL level column default,
and ``required_name`` which is a :class:`.String` of length 30 that's
``NOT NULL``::

    import datetime

    from typing_extensions import Annotated

    from sqlalchemy import func
    from sqlalchemy import String
    from sqlalchemy.orm import mapped_column


    intpk = Annotated[int, mapped_column(primary_key=True)]
    timestamp = Annotated[
        datetime.datetime,
        mapped_column(nullable=False, server_default=func.CURRENT_TIMESTAMP()),
    ]
    required_name = Annotated[str, mapped_column(String(30), nullable=False)]

The above ``Annotated`` objects can then be used directly within
:class:`_orm.Mapped`, where the pre-configured :func:`_orm.mapped_column`
constructs will be extracted and copied to a new instance that will be
specific to each attribute::

    class Base(DeclarativeBase):
        pass


    class SomeClass(Base):
        __tablename__ = "some_table"

        id: Mapped[intpk]
        name: Mapped[required_name]
        created_at: Mapped[timestamp]

``CREATE TABLE`` for our above mapping looks like:

.. sourcecode:: pycon+sql

    >>> from sqlalchemy.schema import CreateTable
    >>> print(CreateTable(SomeClass.__table__))
    {printsql}CREATE TABLE some_table (
      id INTEGER NOT NULL,
      name VARCHAR(30) NOT NULL,
      created_at DATETIME DEFAULT CURRENT_TIMESTAMP NOT NULL,
      PRIMARY KEY (id)
    )

When using ``Annotated`` types in this way, the configuration of the type
may also be affected on a per-attribute basis.  For the types in the above
example that feature explicit use of :paramref:`_orm.mapped_column.nullable`,
we can apply the ``Optional[]`` generic modifier to any of our types so that
the field is optional or not at the Python level, which will be independent
of the ``NULL`` / ``NOT NULL`` setting that takes place in the database::

    from typing_extensions import Annotated

    import datetime
    from typing import Optional

    from sqlalchemy.orm import DeclarativeBase

    timestamp = Annotated[
        datetime.datetime,
        mapped_column(nullable=False),
    ]


    class Base(DeclarativeBase):
        pass


    class SomeClass(Base):
        # ...

        # pep-484 type will be Optional, but column will be
        # NOT NULL
        created_at: Mapped[Optional[timestamp]]

The :func:`_orm.mapped_column` construct is also reconciled with an explicitly
passed :func:`_orm.mapped_column` construct, whose arguments will take precedence
over those of the ``Annotated`` construct.   Below we add a :class:`.ForeignKey`
constraint to our integer primary key and also use an alternate server
default for the ``created_at`` column::

    import datetime

    from typing_extensions import Annotated

    from sqlalchemy import ForeignKey
    from sqlalchemy import func
    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import Mapped
    from sqlalchemy.orm import mapped_column
    from sqlalchemy.schema import CreateTable

    intpk = Annotated[int, mapped_column(primary_key=True)]
    timestamp = Annotated[
        datetime.datetime,
        mapped_column(nullable=False, server_default=func.CURRENT_TIMESTAMP()),
    ]


    class Base(DeclarativeBase):
        pass


    class Parent(Base):
        __tablename__ = "parent"

        id: Mapped[intpk]


    class SomeClass(Base):
        __tablename__ = "some_table"

        # add ForeignKey to mapped_column(Integer, primary_key=True)
        id: Mapped[intpk] = mapped_column(ForeignKey("parent.id"))

        # change server default from CURRENT_TIMESTAMP to UTC_TIMESTAMP
        created_at: Mapped[timestamp] = mapped_column(server_default=func.UTC_TIMESTAMP())

The CREATE TABLE statement illustrates these per-attribute settings,
adding a ``FOREIGN KEY`` constraint as well as substituting
``UTC_TIMESTAMP`` for ``CURRENT_TIMESTAMP``:

.. sourcecode:: pycon+sql

    >>> from sqlalchemy.schema import CreateTable
    >>> print(CreateTable(SomeClass.__table__))
    {printsql}CREATE TABLE some_table (
      id INTEGER NOT NULL,
      created_at DATETIME DEFAULT UTC_TIMESTAMP() NOT NULL,
      PRIMARY KEY (id),
      FOREIGN KEY(id) REFERENCES parent (id)
    )

.. note:: The feature of :func:`_orm.mapped_column` just described, where
   a fully constructed set of column arguments may be indicated using
   :pep:`593` ``Annotated`` objects that contain a "template"
   :func:`_orm.mapped_column` object to be copied into the attribute, is
   currently not implemented for other ORM constructs such as
   :func:`_orm.relationship` and :func:`_orm.composite`.   While this functionality
   is in theory possible, for the moment attempting to use ``Annotated``
   to indicate further arguments for :func:`_orm.relationship` and similar
   will raise a ``NotImplementedError`` exception at runtime, but
   may be implemented in future releases.

.. _orm_declarative_mapped_column_enums:

Using Python ``Enum`` or pep-586 ``Literal`` types in the type map
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 2.0.0b4 - Added ``Enum`` support

.. versionadded:: 2.0.1 - Added ``Literal`` support

User-defined Python types which derive from the Python built-in ``enum.Enum``
as well as the ``typing.Literal``
class are automatically linked to the SQLAlchemy :class:`.Enum` datatype
when used in an ORM declarative mapping.  The example below uses
a custom ``enum.Enum`` within the ``Mapped[]`` constructor::

    import enum

    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import Mapped
    from sqlalchemy.orm import mapped_column


    class Base(DeclarativeBase):
        pass


    class Status(enum.Enum):
        PENDING = "pending"
        RECEIVED = "received"
        COMPLETED = "completed"


    class SomeClass(Base):
        __tablename__ = "some_table"

        id: Mapped[int] = mapped_column(primary_key=True)
        status: Mapped[Status]

In the above example, the mapped attribute ``SomeClass.status`` will be
linked to a :class:`.Column` with the datatype of ``Enum(Status)``.
We can see this for example in the CREATE TABLE output for the PostgreSQL
database:

.. sourcecode:: sql

  CREATE TYPE status AS ENUM ('PENDING', 'RECEIVED', 'COMPLETED')

  CREATE TABLE some_table (
    id SERIAL NOT NULL,
    status status NOT NULL,
    PRIMARY KEY (id)
  )

In a similar way, ``typing.Literal`` may be used instead, using
a ``typing.Literal`` that consists of all strings::


    from typing import Literal

    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import Mapped
    from sqlalchemy.orm import mapped_column


    class Base(DeclarativeBase):
        pass


    Status = Literal["pending", "received", "completed"]


    class SomeClass(Base):
        __tablename__ = "some_table"

        id: Mapped[int] = mapped_column(primary_key=True)
        status: Mapped[Status]

The entries used in :paramref:`_orm.registry.type_annotation_map` link the base
``enum.Enum`` Python type as well as the ``typing.Literal`` type to the
SQLAlchemy :class:`.Enum` SQL type, using a special form which indicates to the
:class:`.Enum` datatype that it should automatically configure itself against
an arbitrary enumerated type. This configuration, which is implicit by default,
would be indicated explicitly as::

    import enum
    import typing

    import sqlalchemy
    from sqlalchemy.orm import DeclarativeBase


    class Base(DeclarativeBase):
        type_annotation_map = {
            enum.Enum: sqlalchemy.Enum(enum.Enum),
            typing.Literal: sqlalchemy.Enum(enum.Enum),
        }

The resolution logic within Declarative is able to resolve subclasses
of ``enum.Enum`` as well as instances of ``typing.Literal`` to match the
``enum.Enum`` or ``typing.Literal`` entry in the
:paramref:`_orm.registry.type_annotation_map` dictionary.  The :class:`.Enum`
SQL type then knows how to produce a configured version of itself with the
appropriate settings, including default string length.   If a ``typing.Literal``
that does not consist of only string values is passed, an informative
error is raised.

Native Enums and Naming
+++++++++++++++++++++++

The :paramref:`.sqltypes.Enum.native_enum` parameter refers to if the
:class:`.sqltypes.Enum` datatype should create a so-called "native"
enum, which on MySQL/MariaDB is the ``ENUM`` datatype and on PostgreSQL is
a new ``TYPE`` object created by ``CREATE TYPE``, or a "non-native" enum,
which means that ``VARCHAR`` will be used to create the datatype.  For
backends other than MySQL/MariaDB or PostgreSQL, ``VARCHAR`` is used in
all cases (third party dialects may have their own behaviors).

Because PostgreSQL's ``CREATE TYPE`` requires that there's an explicit name
for the type to be created, special fallback logic exists when working
with implicitly generated :class:`.sqltypes.Enum` without specifying an
explicit :class:`.sqltypes.Enum` datatype within a mapping:

1. If the :class:`.sqltypes.Enum` is linked to an ``enum.Enum`` object,
   the :paramref:`.sqltypes.Enum.native_enum` parameter defaults to
   ``True`` and the name of the enum will be taken from the name of the
   ``enum.Enum`` datatype.  The PostgreSQL backend will assume ``CREATE TYPE``
   with this name.
2. If the :class:`.sqltypes.Enum` is linked to a ``typing.Literal`` object,
   the :paramref:`.sqltypes.Enum.native_enum` parameter defaults to
   ``False``; no name is generated and ``VARCHAR`` is assumed.

To use ``typing.Literal`` with a PostgreSQL ``CREATE TYPE`` type, an
explicit :class:`.sqltypes.Enum` must be used, either within the
type map::

    import enum
    import typing

    import sqlalchemy
    from sqlalchemy.orm import DeclarativeBase

    Status = Literal["pending", "received", "completed"]


    class Base(DeclarativeBase):
        type_annotation_map = {
            Status: sqlalchemy.Enum("pending", "received", "completed", name="status_enum"),
        }

Or alternatively within :func:`_orm.mapped_column`::

    import enum
    import typing

    import sqlalchemy
    from sqlalchemy.orm import DeclarativeBase

    Status = Literal["pending", "received", "completed"]


    class Base(DeclarativeBase):
        pass


    class SomeClass(Base):
        __tablename__ = "some_table"

        id: Mapped[int] = mapped_column(primary_key=True)
        status: Mapped[Status] = mapped_column(
            sqlalchemy.Enum("pending", "received", "completed", name="status_enum")
        )

Altering the Configuration of the Default Enum
+++++++++++++++++++++++++++++++++++++++++++++++

In order to modify the fixed configuration of the :class:`.enum.Enum` datatype
that's generated implicitly, specify new entries in the
:paramref:`_orm.registry.type_annotation_map`, indicating additional arguments.
For example, to use "non native enumerations" unconditionally, the
:paramref:`.Enum.native_enum` parameter may be set to False for all types::

    import enum
    import typing
    import sqlalchemy
    from sqlalchemy.orm import DeclarativeBase


    class Base(DeclarativeBase):
        type_annotation_map = {
            enum.Enum: sqlalchemy.Enum(enum.Enum, native_enum=False),
            typing.Literal: sqlalchemy.Enum(enum.Enum, native_enum=False),
        }

.. versionchanged:: 2.0.1  Implemented support for overriding parameters
   such as :paramref:`_sqltypes.Enum.native_enum` within the
   :class:`_sqltypes.Enum` datatype when establishing the
   :paramref:`_orm.registry.type_annotation_map`.  Previously, this
   functionality was not working.

To use a specific configuration for a specific ``enum.Enum`` subtype, such
as setting the string length to 50 when using the example ``Status``
datatype::

    import enum
    import sqlalchemy
    from sqlalchemy.orm import DeclarativeBase


    class Status(enum.Enum):
        PENDING = "pending"
        RECEIVED = "received"
        COMPLETED = "completed"


    class Base(DeclarativeBase):
        type_annotation_map = {
            Status: sqlalchemy.Enum(Status, length=50, native_enum=False)
        }

By default :class:`_sqltypes.Enum` that are automatically generated are not
associated with the :class:`_sql.MetaData` instance used by the ``Base``, so if
the metadata defines a schema it will not be automatically associated with the
enum. To automatically associate the enum with the schema in the metadata or
table they belong to the :paramref:`_sqltypes.Enum.inherit_schema` can be set::

    from enum import Enum
    import sqlalchemy as sa
    from sqlalchemy.orm import DeclarativeBase


    class Base(DeclarativeBase):
        metadata = sa.MetaData(schema="my_schema")
        type_annotation_map = {Enum: sa.Enum(Enum, inherit_schema=True)}

Linking Specific ``enum.Enum`` or ``typing.Literal`` to other datatypes
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

The above examples feature the use of an :class:`_sqltypes.Enum` that is
automatically configuring itself to the arguments / attributes present on
an ``enum.Enum`` or ``typing.Literal`` type object.    For use cases where
specific kinds of ``enum.Enum`` or ``typing.Literal`` should be linked to
other types, these specific types may be placed in the type map also.
In the example below, an entry for ``Literal[]`` that contains non-string
types is linked to the :class:`_sqltypes.JSON` datatype::


    from typing import Literal

    from sqlalchemy import JSON
    from sqlalchemy.orm import DeclarativeBase

    my_literal = Literal[0, 1, True, False, "true", "false"]


    class Base(DeclarativeBase):
        type_annotation_map = {my_literal: JSON}

In the above configuration, the ``my_literal`` datatype will resolve to a
:class:`._sqltypes.JSON` instance.  Other ``Literal`` variants will continue
to resolve to :class:`_sqltypes.Enum` datatypes.


Dataclass features in ``mapped_column()``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The :func:`_orm.mapped_column` construct integrates with SQLAlchemy's
"native dataclasses" feature, discussed at
:ref:`orm_declarative_native_dataclasses`.   See that section for current
background on additional directives supported by :func:`_orm.mapped_column`.



.. _orm_declarative_metadata:

Accessing Table and Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A declaratively mapped class will always include an attribute called
``__table__``; when the above configuration using ``__tablename__`` is
complete, the declarative process makes the :class:`_schema.Table`
available via the ``__table__`` attribute::


    # access the Table
    user_table = User.__table__

The above table is ultimately the same one that corresponds to the
:attr:`_orm.Mapper.local_table` attribute, which we can see through the
:ref:`runtime inspection system <inspection_toplevel>`::

    from sqlalchemy import inspect

    user_table = inspect(User).local_table

The :class:`_schema.MetaData` collection associated with both the declarative
:class:`_orm.registry` as well as the base class is frequently necessary in
order to run DDL operations such as CREATE, as well as in use with migration
tools such as Alembic.   This object is available via the ``.metadata``
attribute of :class:`_orm.registry` as well as the declarative base class.
Below, for a small script we may wish to emit a CREATE for all tables against a
SQLite database::

    engine = create_engine("sqlite://")

    Base.metadata.create_all(engine)

.. _orm_declarative_table_configuration:

Declarative Table Configuration
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When using Declarative Table configuration with the ``__tablename__``
declarative class attribute, additional arguments to be supplied to the
:class:`_schema.Table` constructor should be provided using the
``__table_args__`` declarative class attribute.

This attribute accommodates both positional as well as keyword
arguments that are normally sent to the
:class:`_schema.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},
        )

A class may also specify the ``__table_args__`` declarative attribute,
as well as the ``__tablename__`` attribute, in a dynamic style using the
:func:`_orm.declared_attr` method decorator.   See
:ref:`orm_mixins_toplevel` for background.

.. _orm_declarative_table_schema_name:

Explicit Schema Name with Declarative Table
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The schema name for a :class:`_schema.Table` as documented at
:ref:`schema_table_schema_name` is applied to an individual :class:`_schema.Table`
using the :paramref:`_schema.Table.schema` argument.   When using Declarative
tables, this option is passed like any other to the ``__table_args__``
dictionary::

    from sqlalchemy.orm import DeclarativeBase


    class Base(DeclarativeBase):
        pass


    class MyClass(Base):
        __tablename__ = "sometable"
        __table_args__ = {"schema": "some_schema"}

The schema name can also be applied to all :class:`_schema.Table` objects
globally by using the :paramref:`_schema.MetaData.schema` parameter documented
at :ref:`schema_metadata_schema_name`.   The :class:`_schema.MetaData` object
may be constructed separately and associated with a :class:`_orm.DeclarativeBase`
subclass by assigning to the ``metadata`` attribute directly::

    from sqlalchemy import MetaData
    from sqlalchemy.orm import DeclarativeBase

    metadata_obj = MetaData(schema="some_schema")


    class Base(DeclarativeBase):
        metadata = metadata_obj


    class MyClass(Base):
        # will use "some_schema" by default
        __tablename__ = "sometable"

.. seealso::

    :ref:`schema_table_schema_name` - in the :ref:`metadata_toplevel` documentation.

.. _orm_declarative_column_options:

Setting Load and Persistence Options for Declarative Mapped Columns
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The :func:`_orm.mapped_column` construct accepts additional ORM-specific
arguments that affect how the generated :class:`_schema.Column` is
mapped, affecting its load and persistence-time behavior.   Options
that are commonly used include:

* **deferred column loading** - The :paramref:`_orm.mapped_column.deferred`
  boolean establishes the :class:`_schema.Column` using
  :ref:`deferred column loading <orm_queryguide_column_deferral>` by default.  In the example
  below, the ``User.bio`` column will not be loaded by default, but only
  when accessed::

      class User(Base):
          __tablename__ = "user"

          id: Mapped[int] = mapped_column(primary_key=True)
          name: Mapped[str]
          bio: Mapped[str] = mapped_column(Text, deferred=True)

  .. seealso::

     :ref:`orm_queryguide_column_deferral` - full description of deferred column loading

* **active history** - The :paramref:`_orm.mapped_column.active_history`
  ensures that upon change of value for the attribute, the previous value
  will have been loaded and made part of the :attr:`.AttributeState.history`
  collection when inspecting the history of the attribute.   This may incur
  additional SQL statements::

    class User(Base):
        __tablename__ = "user"

        id: Mapped[int] = mapped_column(primary_key=True)
        important_identifier: Mapped[str] = mapped_column(active_history=True)

See the docstring for :func:`_orm.mapped_column` for a list of supported
parameters.

.. seealso::

    :ref:`orm_imperative_table_column_options` - describes using
    :func:`_orm.column_property` and :func:`_orm.deferred` for use with
    Imperative Table configuration

.. _mapper_column_distinct_names:

.. _orm_declarative_table_column_naming:

Naming Declarative Mapped Columns Explicitly
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

All of the examples thus far feature the :func:`_orm.mapped_column` construct
linked to an ORM mapped attribute, where the Python attribute name given
to the :func:`_orm.mapped_column` is also that of the column as we see in
CREATE TABLE statements as well as queries.   The name for a column as
expressed in SQL may be indicated by passing the string positional argument
:paramref:`_orm.mapped_column.__name` as the first positional argument.
In the example below, the ``User`` class is mapped with alternate names
given to the columns themselves::

    class User(Base):
        __tablename__ = "user"

        id: Mapped[int] = mapped_column("user_id", primary_key=True)
        name: Mapped[str] = mapped_column("user_name")

Where above ``User.id`` resolves to a column named ``user_id``
and ``User.name`` resolves to a column named ``user_name``.  We
may write a :func:`_sql.select` statement using our Python attribute names
and will see the SQL names generated:

.. sourcecode:: pycon+sql

    >>> from sqlalchemy import select
    >>> print(select(User.id, User.name).where(User.name == "x"))
    {printsql}SELECT "user".user_id, "user".user_name
    FROM "user"
    WHERE "user".user_name = :user_name_1


.. seealso::

    :ref:`orm_imperative_table_column_naming` - applies to Imperative Table

.. _orm_declarative_table_adding_columns:

Appending additional columns to an existing Declarative mapped class
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A declarative table configuration allows the addition of new
:class:`_schema.Column` objects to an existing mapping after the :class:`.Table`
metadata has already been generated.

For a declarative class that is declared using a declarative base class,
the underlying metaclass :class:`.DeclarativeMeta` includes a ``__setattr__()``
method that will intercept additional :func:`_orm.mapped_column` or Core
:class:`.Column` objects and
add them to both the :class:`.Table` using :meth:`.Table.append_column`
as well as to the existing :class:`.Mapper` using :meth:`.Mapper.add_property`::

    MyClass.some_new_column = mapped_column(String)

Using core :class:`_schema.Column`::

    MyClass.some_new_column = Column(String)

All arguments are supported including an alternate name, such as
``MyClass.some_new_column = mapped_column("some_name", String)``.  However,
the SQL type must be passed to the :func:`_orm.mapped_column` or
:class:`_schema.Column` object explicitly, as in the above examples where
the :class:`_sqltypes.String` type is passed.  There's no capability for
the :class:`_orm.Mapped` annotation type to take part in the operation.

Additional :class:`_schema.Column` objects may also be added to a mapping
in the specific circumstance of using single table inheritance, where
additional columns are present on mapped subclasses that have
no :class:`.Table` of their own.  This is illustrated in the section
:ref:`single_inheritance`.

.. seealso::

   :ref:`orm_declarative_table_adding_relationship` - similar examples for :func:`_orm.relationship`

.. note:: Assignment of mapped
    properties to an already mapped class will only
    function correctly if the "declarative base" class is used, meaning
    the user-defined subclass of :class:`_orm.DeclarativeBase` or the
    dynamically generated class returned by :func:`_orm.declarative_base`
    or :meth:`_orm.registry.generate_base`.   This "base" class includes
    a Python metaclass which implements a special ``__setattr__()`` method
    that intercepts these operations.

    Runtime assignment of class-mapped attributes to a mapped class will **not** work
    if the class is mapped using decorators like :meth:`_orm.registry.mapped`
    or imperative functions like :meth:`_orm.registry.map_imperatively`.


.. _orm_imperative_table_configuration:

Declarative with Imperative Table (a.k.a. Hybrid Declarative)
-------------------------------------------------------------

Declarative mappings may also be provided with a pre-existing
:class:`_schema.Table` object, or otherwise a :class:`_schema.Table` or other
arbitrary :class:`_sql.FromClause` construct (such as a :class:`_sql.Join`
or :class:`_sql.Subquery`) that is constructed separately.

This is referred to as a "hybrid declarative"
mapping, as the class is mapped using the declarative style for everything
involving the mapper configuration, however the mapped :class:`_schema.Table`
object is produced separately and passed to the declarative process
directly::


    from sqlalchemy import Column, ForeignKey, Integer, String
    from sqlalchemy.orm import DeclarativeBase


    class Base(DeclarativeBase):
        pass


    # construct a Table directly.  The Base.metadata collection is
    # usually a good choice for MetaData but any MetaData
    # collection may be used.

    user_table = Table(
        "user",
        Base.metadata,
        Column("id", Integer, primary_key=True),
        Column("name", String),
        Column("fullname", String),
        Column("nickname", String),
    )


    # construct the User class using this table.
    class User(Base):
        __table__ = user_table

Above, a :class:`_schema.Table` object is constructed using the approach
described at :ref:`metadata_describing`.   It can then be applied directly
to a class that is declaratively mapped.  The ``__tablename__`` and
``__table_args__`` declarative class attributes are not used in this form.
The above configuration is often more readable as an inline definition::

    class User(Base):
        __table__ = Table(
            "user",
            Base.metadata,
            Column("id", Integer, primary_key=True),
            Column("name", String),
            Column("fullname", String),
            Column("nickname", String),
        )

A natural effect of the above style is that the ``__table__`` attribute is
itself defined within the class definition block.   As such it may be
immediately referenced within subsequent attributes, such as the example
below which illustrates referring to the ``type`` column in a polymorphic
mapper configuration::

    class Person(Base):
        __table__ = Table(
            "person",
            Base.metadata,
            Column("id", Integer, primary_key=True),
            Column("name", String(50)),
            Column("type", String(50)),
        )

        __mapper_args__ = {
            "polymorphic_on": __table__.c.type,
            "polymorhpic_identity": "person",
        }

The "imperative table" form is also used when a non-:class:`_schema.Table`
construct, such as a :class:`_sql.Join` or :class:`_sql.Subquery` object,
is to be mapped.  An example below::

    from sqlalchemy import func, select

    subq = (
        select(
            func.count(orders.c.id).label("order_count"),
            func.max(orders.c.price).label("highest_order"),
            orders.c.customer_id,
        )
        .group_by(orders.c.customer_id)
        .subquery()
    )

    customer_select = (
        select(customers, subq)
        .join_from(customers, subq, customers.c.id == subq.c.customer_id)
        .subquery()
    )


    class Customer(Base):
        __table__ = customer_select

For background on mapping to non-:class:`_schema.Table` constructs see
the sections :ref:`orm_mapping_joins` and :ref:`orm_mapping_arbitrary_subqueries`.

The "imperative table" form is of particular use when the class itself
is using an alternative form of attribute declaration, such as Python
dataclasses.   See the section :ref:`orm_declarative_dataclasses` for detail.

.. seealso::

    :ref:`metadata_describing`

    :ref:`orm_declarative_dataclasses`

.. _orm_imperative_table_column_naming:

Alternate Attribute Names for Mapping Table Columns
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The section :ref:`orm_declarative_table_column_naming` illustrated how to
use :func:`_orm.mapped_column` to provide a specific name for the generated
:class:`_schema.Column` object separate from the attribute name under which
it is mapped.

When using Imperative Table configuration, we already have
:class:`_schema.Column` objects present.  To map these to alternate names
we may assign the :class:`_schema.Column` to the desired attributes
directly::

    user_table = Table(
        "user",
        Base.metadata,
        Column("user_id", Integer, primary_key=True),
        Column("user_name", String),
    )


    class User(Base):
        __table__ = user_table

        id = user_table.c.user_id
        name = user_table.c.user_name

The ``User`` mapping above will refer to the ``"user_id"`` and ``"user_name"``
columns via the ``User.id`` and ``User.name`` attributes, in the same
way as demonstrated at :ref:`orm_declarative_table_column_naming`.

One caveat to the above mapping is that the direct inline link to
:class:`_schema.Column` will not be typed correctly when using
:pep:`484` typing tools.   A strategy to resolve this is to apply the
:class:`_schema.Column` objects within the :func:`_orm.column_property`
function; while the :class:`_orm.Mapper` already generates this property
object for its internal use automatically, by naming it in the class
declaration, typing tools will be able to match the attribute to the
:class:`_orm.Mapped` annotation::

    from sqlalchemy.orm import column_property
    from sqlalchemy.orm import Mapped


    class User(Base):
        __table__ = user_table

        id: Mapped[int] = column_property(user_table.c.user_id)
        name: Mapped[str] = column_property(user_table.c.user_name)

.. seealso::

    :ref:`orm_declarative_table_column_naming` - applies to Declarative Table

.. _column_property_options:

.. _orm_imperative_table_column_options:

Applying Load, Persistence and Mapping Options for Imperative Table Columns
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The section :ref:`orm_declarative_column_options` reviewed how to set load
and persistence options when using the :func:`_orm.mapped_column` construct
with Declarative Table configuration.  When using Imperative Table configuration,
we already have existing :class:`_schema.Column` objects that are mapped.
In order to map these :class:`_schema.Column` objects along with additional
parameters that are specific to the ORM mapping, we may use the
:func:`_orm.column_property` and :func:`_orm.deferred` constructs in order to
associate additional parameters with the column.   Options include:

* **deferred column loading** - The :func:`_orm.deferred` function is shorthand
  for invoking :func:`_orm.column_property` with the
  :paramref:`_orm.column_property.deferred` parameter set to ``True``;
  this construct establishes the :class:`_schema.Column` using
  :ref:`deferred column loading <orm_queryguide_column_deferral>` by default.  In the example
  below, the ``User.bio`` column will not be loaded by default, but only
  when accessed::

    from sqlalchemy.orm import deferred

    user_table = Table(
        "user",
        Base.metadata,
        Column("id", Integer, primary_key=True),
        Column("name", String),
        Column("bio", Text),
    )


    class User(Base):
        __table__ = user_table

        bio = deferred(user_table.c.bio)

 .. seealso::

     :ref:`orm_queryguide_column_deferral` - full description of deferred column loading

* **active history** - The :paramref:`_orm.column_property.active_history`
  ensures that upon change of value for the attribute, the previous value
  will have been loaded and made part of the :attr:`.AttributeState.history`
  collection when inspecting the history of the attribute.   This may incur
  additional SQL statements::

    from sqlalchemy.orm import deferred

    user_table = Table(
        "user",
        Base.metadata,
        Column("id", Integer, primary_key=True),
        Column("important_identifier", String),
    )


    class User(Base):
        __table__ = user_table

        important_identifier = column_property(
            user_table.c.important_identifier, active_history=True
        )

.. seealso::

    The :func:`_orm.column_property` construct is also important for cases
    where classes are mapped to alternative FROM clauses such as joins and
    selects.  More background on these cases is at:

    * :ref:`maptojoin`

    * :ref:`mapper_sql_expressions`

    For Declarative Table configuration with :func:`_orm.mapped_column`,
    most options are available directly; see the section
    :ref:`orm_declarative_column_options` for examples.



.. _orm_declarative_reflected:

Mapping Declaratively with Reflected Tables
--------------------------------------------

There are several patterns available which provide for producing mapped
classes against a series of :class:`_schema.Table` objects that were
introspected from the database, using the reflection process described at
:ref:`metadata_reflection`.

A simple way to map a class to a table reflected from the database is to
use a declarative hybrid mapping, passing the
:paramref:`_schema.Table.autoload_with` parameter to the constructor for
:class:`_schema.Table`::

    from sqlalchemy import create_engine
    from sqlalchemy import Table
    from sqlalchemy.orm import DeclarativeBase

    engine = create_engine("postgresql+psycopg2://user:pass@hostname/my_existing_database")


    class Base(DeclarativeBase):
        pass


    class MyClass(Base):
        __table__ = Table(
            "mytable",
            Base.metadata,
            autoload_with=engine,
        )

A variant on the above pattern that scales for many tables is to use the
:meth:`.MetaData.reflect` method to reflect a full set of :class:`.Table`
objects at once, then refer to them from the :class:`.MetaData`::


    from sqlalchemy import create_engine
    from sqlalchemy import Table
    from sqlalchemy.orm import DeclarativeBase

    engine = create_engine("postgresql+psycopg2://user:pass@hostname/my_existing_database")


    class Base(DeclarativeBase):
        pass


    Base.metadata.reflect(engine)


    class MyClass(Base):
        __table__ = Base.metadata.tables["mytable"]

One caveat to the approach of using ``__table__`` is that the mapped classes cannot
be declared until the tables have been reflected, which requires the database
connectivity source to be present while the application classes are being
declared; it's typical that classes are declared as the modules of an
application are being imported, but database connectivity isn't available
until the application starts running code so that it can consume configuration
information and create an engine.   There are currently two approaches
to working around this, described in the next two sections.

.. _orm_declarative_reflected_deferred_reflection:

Using DeferredReflection
^^^^^^^^^^^^^^^^^^^^^^^^^

To accommodate the use case of declaring mapped classes where reflection of
table metadata can occur afterwards, a simple extension called the
:class:`.DeferredReflection` mixin is available, which alters the declarative
mapping process to be delayed until a special class-level
:meth:`.DeferredReflection.prepare` method is called, which will perform
the reflection process against a target database, and will integrate the
results with the declarative table mapping process, that is, classes which
use the ``__tablename__`` attribute::

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


    class Base(DeclarativeBase):
        pass


    class Reflected(DeferredReflection):
        __abstract__ = True


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


    class Bar(Reflected, Base):
        __tablename__ = "bar"

        foo_id = mapped_column(Integer, ForeignKey("foo.id"))

Above, we create a mixin class ``Reflected`` that will serve as a base
for classes in our declarative hierarchy that should become mapped when
the ``Reflected.prepare`` method is called.   The above mapping is not
complete until we do so, given an :class:`_engine.Engine`::


    engine = create_engine("postgresql+psycopg2://user:pass@hostname/my_existing_database")
    Reflected.prepare(engine)

The purpose of the ``Reflected`` class is to define the scope at which
classes should be reflectively mapped.   The plugin will search among the
subclass tree of the target against which ``.prepare()`` is called and reflect
all tables which are named by declared classes; tables in the target database
that are not part of mappings and are not related to the target tables
via foreign key constraint will not be reflected.

Using Automap
^^^^^^^^^^^^^^

A more automated solution to mapping against an existing database where table
reflection is to be used is to use the :ref:`automap_toplevel` extension. This
extension will generate entire mapped classes from a database schema, including
relationships between classes based on observed foreign key constraints. While
it includes hooks for customization, such as hooks that allow custom
class naming and relationship naming schemes, automap is oriented towards an
expedient zero-configuration style of working. If an application wishes to have
a fully explicit model that makes use of table reflection, the
:ref:`DeferredReflection <orm_declarative_reflected_deferred_reflection>`
class may be preferable for its less automated approach.

.. seealso::

    :ref:`automap_toplevel`


.. _mapper_automated_reflection_schemes:

Automating Column Naming Schemes from Reflected Tables
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When using any of the previous reflection techniques, we have the option
to change the naming scheme by which columns are mapped.   The
:class:`_schema.Column` object includes a parameter
:paramref:`_schema.Column.key` which is a string name that determines
under what name
this :class:`_schema.Column` will be present in the :attr:`_schema.Table.c`
collection, independently of the SQL name of the column.  This key is also
used by :class:`_orm.Mapper` as the attribute name under which the
:class:`_schema.Column` will be mapped, if not supplied through other
means such as that illustrated at :ref:`orm_imperative_table_column_naming`.

When working with table reflection, we can intercept the parameters that
will be used for :class:`_schema.Column` as they are received using
the :meth:`_events.DDLEvents.column_reflect` event and apply whatever
changes we need, including the ``.key`` attribute but also things like
datatypes.

The event hook is most easily
associated with the :class:`_schema.MetaData` object that's in use
as illustrated below::

    from sqlalchemy import event
    from sqlalchemy.orm import DeclarativeBase


    class Base(DeclarativeBase):
        pass


    @event.listens_for(Base.metadata, "column_reflect")
    def column_reflect(inspector, table, column_info):
        # set column.key = "attr_<lower_case_name>"
        column_info["key"] = "attr_%s" % column_info["name"].lower()

With the above event, the reflection of :class:`_schema.Column` objects will be intercepted
with our event that adds a new ".key" element, such as in a mapping as below::

    class MyClass(Base):
        __table__ = Table("some_table", Base.metadata, autoload_with=some_engine)

The approach also works with both the :class:`.DeferredReflection` base class
as well as with the :ref:`automap_toplevel` extension.   For automap
specifically, see the section :ref:`automap_intercepting_columns` for
background.

.. seealso::

    :ref:`orm_declarative_reflected`

    :meth:`_events.DDLEvents.column_reflect`

    :ref:`automap_intercepting_columns` - in the :ref:`automap_toplevel` documentation


.. _mapper_primary_key:

Mapping to an Explicit Set of Primary Key Columns
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The :class:`.Mapper` construct in order to successfully map a table always
requires that at least one column be identified as the "primary key" for
that selectable.   This is so that when an ORM object is loaded or persisted,
it can be placed in the :term:`identity map` with an appropriate
:term:`identity key`.

In those cases where the a reflected table to be mapped does not include
a primary key constraint, as well as in the general case for
:ref:`mapping against arbitrary selectables <orm_mapping_arbitrary_subqueries>`
where primary key columns might not be present, the
:paramref:`.Mapper.primary_key` parameter is provided so that any set of
columns may be configured as the "primary key" for the table, as far as
ORM mapping is concerned.

Given the following example of an Imperative Table
mapping against an existing :class:`.Table` object where the table does not
have any declared primary key (as may occur in reflection scenarios), we may
map such a table as in the following example::

    from sqlalchemy import Column
    from sqlalchemy import MetaData
    from sqlalchemy import String
    from sqlalchemy import Table
    from sqlalchemy import UniqueConstraint
    from sqlalchemy.orm import DeclarativeBase


    metadata = MetaData()
    group_users = Table(
        "group_users",
        metadata,
        Column("user_id", String(40), nullable=False),
        Column("group_id", String(40), nullable=False),
        UniqueConstraint("user_id", "group_id"),
    )


    class Base(DeclarativeBase):
        pass


    class GroupUsers(Base):
        __table__ = group_users
        __mapper_args__ = {"primary_key": [group_users.c.user_id, group_users.c.group_id]}

Above, the ``group_users`` table is an association table of some kind
with string columns ``user_id`` and ``group_id``, but no primary key is set up;
instead, there is only a :class:`.UniqueConstraint` establishing that the
two columns represent a unique key.   The :class:`.Mapper` does not automatically
inspect unique constraints for primary keys; instead, we make use of the
:paramref:`.Mapper.primary_key` parameter, passing a collection of
``[group_users.c.user_id, group_users.c.group_id]``, indicating that these two
columns should be used in order to construct the identity key for instances
of the ``GroupUsers`` class.


.. _include_exclude_cols:

Mapping a Subset of Table Columns
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Sometimes table reflection may provide a :class:`_schema.Table` with many
columns that are not important for our needs and may be safely ignored.
For such a table that has lots of columns that don't need to be referenced
in the application, the :paramref:`_orm.Mapper.include_properties`
or :paramref:`_orm.Mapper.exclude_properties` parameters can indicate
a subset of columns to be mapped, where other columns from the
target :class:`_schema.Table` will not be considered by the ORM in any
way.   Example::

    class User(Base):
        __table__ = user_table
        __mapper_args__ = {"include_properties": ["user_id", "user_name"]}

In the above example, the ``User`` class will map to the ``user_table`` table, only
including the ``user_id`` and ``user_name`` columns - the rest are not referenced.

Similarly::

    class Address(Base):
        __table__ = address_table
        __mapper_args__ = {"exclude_properties": ["street", "city", "state", "zip"]}

will map the ``Address`` class to the ``address_table`` table, including
all columns present except ``street``, ``city``, ``state``, and ``zip``.

As indicated in the two examples, columns may be referenced either
by string name or by referring to the :class:`_schema.Column` object
directly.  Referring to the object directly may be useful for explicitness as
well as to resolve ambiguities when
mapping to multi-table constructs that might have repeated names::

    class User(Base):
        __table__ = user_table
        __mapper_args__ = {
            "include_properties": [user_table.c.user_id, user_table.c.user_name]
        }

When columns are not included in a mapping, these columns will not be
referenced in any SELECT statements emitted when executing :func:`_sql.select`
or legacy :class:`_query.Query` objects, nor will there be any mapped attribute
on the mapped class which represents the column; assigning an attribute of that
name will have no effect beyond that of a normal Python attribute assignment.

However, it is important to note that **schema level column defaults WILL
still be in effect** for those :class:`_schema.Column` objects that include them,
even though they may be excluded from the ORM mapping.

"Schema level column defaults" refers to the defaults described at
:ref:`metadata_defaults` including those configured by the
:paramref:`_schema.Column.default`, :paramref:`_schema.Column.onupdate`,
:paramref:`_schema.Column.server_default` and
:paramref:`_schema.Column.server_onupdate` parameters. These constructs
continue to have normal effects because in the case of
:paramref:`_schema.Column.default` and :paramref:`_schema.Column.onupdate`, the
:class:`_schema.Column` object is still present on the underlying
:class:`_schema.Table`, thus allowing the default functions to take place when
the ORM emits an INSERT or UPDATE, and in the case of
:paramref:`_schema.Column.server_default` and
:paramref:`_schema.Column.server_onupdate`, the relational database itself
emits these defaults as a server side behavior.



.. _mypy: https://mypy.readthedocs.io/en/stable/

.. _pylance: https://github.com/microsoft/pylance-release

.. _generic: https://peps.python.org/pep-0484/#generics

.. _dataclasses: https://docs.python.org/3/library/dataclasses.html
