=====================
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

    In general constraints are **not** checked during ``full_clean()``, and do
    not raise ``ValidationError``\s. Rather you'll get a database integrity
    error on ``save()``. ``UniqueConstraint``\s without a
    :attr:`~UniqueConstraint.condition` (i.e. non-partial unique constraints)
    are different in this regard, in that they leverage the existing
    ``validate_unique()`` logic, and thus enable two-stage validation. In
    addition to ``IntegrityError`` on ``save()``, ``ValidationError`` is also
    raised during model validation when the ``UniqueConstraint`` is violated.

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

.. class:: CheckConstraint(*, check, name)

    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:: 3.1

    Support for boolean :class:`~django.db.models.Expression` was added.

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

.. attribute:: CheckConstraint.name

The name of the constraint.

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

.. class:: UniqueConstraint(*, fields, name, condition=None, deferrable=None)

    Creates a unique constraint in the database.

``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.

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

.. attribute:: UniqueConstraint.name

The name of the constraint.

``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

.. versionadded:: 3.1

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>`_.