=====================
Constraints reference
=====================

.. module:: django.db.models.constraints

.. currentmodule:: django.db.models

The classes defined in this module create database constraints. They are added
in the model :attr:`Meta.constraints <django.db.models.Options.constraints>`
option.

.. admonition:: Referencing built-in constraints

    Constraints are defined in ``django.db.models.constraints``, but for
    convenience they're imported into :mod:`django.db.models`. The standard
    convention is to use ``from django.db import models`` and refer to the
    constraints as ``models.<Foo>Constraint``.

.. admonition:: Constraints in abstract base classes

    You must always specify a unique name for the constraint. As such, you
    cannot normally specify a constraint on an abstract base class, since the
    :attr:`Meta.constraints <django.db.models.Options.constraints>` option is
    inherited by subclasses, with exactly the same values for the attributes
    (including ``name``) each time. To work around name collisions, part of the
    name may contain ``'%(app_label)s'`` and ``'%(class)s'``, which are
    replaced, respectively, by the lowercased app label and class name of the
    concrete model. For example ``CheckConstraint(check=Q(age__gte=18),
    name='%(app_label)s_%(class)s_is_adult')``.

.. admonition:: Validation of Constraints

    Constraints are checked during the :ref:`model validation
    <validating-objects>`.

.. versionchanged:: 4.1

    In older versions, constraints were not checked during model validation.

``BaseConstraint``
==================

.. class:: BaseConstraint(name, violation_error_message=None)

    Base class for all constraints. Subclasses must implement
    ``constraint_sql()``, ``create_sql()``, ``remove_sql()`` and
    ``validate()`` methods.

All constraints have the following parameters in common:

``name``
--------

.. attribute:: BaseConstraint.name

The name of the constraint. You must always specify a unique name for the
constraint.

``violation_error_message``
---------------------------

.. versionadded:: 4.1

.. attribute:: BaseConstraint.violation_error_message

The error message used when ``ValidationError`` is raised during
:ref:`model validation <validating-objects>`. Defaults to
``"Constraint “%(name)s” is violated."``.

``validate()``
--------------

.. versionadded:: 4.1

.. method:: BaseConstraint.validate(model, instance, exclude=None, using=DEFAULT_DB_ALIAS)

Validates that the constraint, defined on ``model``, is respected on the
``instance``. This will do a query on the database to ensure that the
constraint is respected. If fields in the ``exclude`` list are needed to
validate the constraint, the constraint is ignored.

Raise a ``ValidationError`` if the constraint is violated.

This method must be implemented by a subclass.

``CheckConstraint``
===================

.. class:: CheckConstraint(*, check, name, violation_error_message=None)

    Creates a check constraint in the database.

``check``
---------

.. attribute:: CheckConstraint.check

A :class:`Q` object or boolean :class:`~django.db.models.Expression` that
specifies the check you want the constraint to enforce.

For example, ``CheckConstraint(check=Q(age__gte=18), name='age_gte_18')``
ensures the age field is never less than 18.

.. versionchanged:: 4.1

    The ``violation_error_message`` argument was added.

``UniqueConstraint``
====================

.. class:: UniqueConstraint(*expressions, fields=(), name=None, condition=None, deferrable=None, include=None, opclasses=(), violation_error_message=None)

    Creates a unique constraint in the database.

``expressions``
---------------

.. attribute:: UniqueConstraint.expressions

Positional argument ``*expressions`` allows creating functional unique
constraints on expressions and database functions.

For example::

    UniqueConstraint(Lower('name').desc(), 'category', name='unique_lower_name_category')

creates a unique constraint on the lowercased value of the ``name`` field in
descending order and the ``category`` field in the default ascending order.

Functional unique constraints have the same database restrictions as
:attr:`Index.expressions`.

``fields``
----------

.. attribute:: UniqueConstraint.fields

A list of field names that specifies the unique set of columns you want the
constraint to enforce.

For example, ``UniqueConstraint(fields=['room', 'date'],
name='unique_booking')`` ensures each room can only be booked once for each
date.

``condition``
-------------

.. attribute:: UniqueConstraint.condition

A :class:`Q` object that specifies the condition you want the constraint to
enforce.

For example::

    UniqueConstraint(fields=['user'], condition=Q(status='DRAFT'), name='unique_draft_user')

ensures that each user only has one draft.

These conditions have the same database restrictions as
:attr:`Index.condition`.

``deferrable``
--------------

.. attribute:: UniqueConstraint.deferrable

Set this parameter to create a deferrable unique constraint. Accepted values
are ``Deferrable.DEFERRED`` or ``Deferrable.IMMEDIATE``. For example::

    from django.db.models import Deferrable, UniqueConstraint

    UniqueConstraint(
        name='unique_order',
        fields=['order'],
        deferrable=Deferrable.DEFERRED,
    )

By default constraints are not deferred. A deferred constraint will not be
enforced until the end of the transaction. An immediate constraint will be
enforced immediately after every command.

.. admonition:: MySQL, MariaDB, and SQLite.

    Deferrable unique constraints are ignored on MySQL, MariaDB, and SQLite as
    neither supports them.

.. warning::

    Deferred unique constraints may lead to a `performance penalty
    <https://www.postgresql.org/docs/current/sql-createtable.html#id-1.9.3.85.9.4>`_.

``include``
-----------

.. attribute:: UniqueConstraint.include

A list or tuple of the names of the fields to be included in the covering
unique index as non-key columns. This allows index-only scans to be used for
queries that select only included fields (:attr:`~UniqueConstraint.include`)
and filter only by unique fields (:attr:`~UniqueConstraint.fields`).

For example::

    UniqueConstraint(name='unique_booking', fields=['room', 'date'], include=['full_name'])

will allow filtering on ``room`` and ``date``, also selecting ``full_name``,
while fetching data only from the index.

``include`` is supported only on PostgreSQL.

Non-key columns have the same database restrictions as :attr:`Index.include`.


``opclasses``
-------------

.. attribute:: UniqueConstraint.opclasses

The names of the `PostgreSQL operator classes
<https://www.postgresql.org/docs/current/indexes-opclass.html>`_ to use for
this unique index. If you require a custom operator class, you must provide one
for each field in the index.

For example::

    UniqueConstraint(name='unique_username', fields=['username'], opclasses=['varchar_pattern_ops'])

creates a unique index on ``username`` using ``varchar_pattern_ops``.

``opclasses`` are ignored for databases besides PostgreSQL.

``violation_error_message``
---------------------------

.. versionadded:: 4.1

.. attribute:: UniqueConstraint.violation_error_message

The error message used when ``ValidationError`` is raised during
:ref:`model validation <validating-objects>`. Defaults to
:attr:`.BaseConstraint.violation_error_message`.

This message is *not used* for :class:`UniqueConstraint`\s with
:attr:`~UniqueConstraint.fields` and without a
:attr:`~UniqueConstraint.condition`. Such :class:`~UniqueConstraint`\s show the
same message as constraints defined with
:attr:`.Field.unique` or in
:attr:`Meta.unique_together <django.db.models.Options.constraints>`.
