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=====================
Constraints reference
=====================
.. module:: django.db.models.constraints
.. currentmodule:: django.db.models
.. versionadded:: 2.2
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. Instead, specify the ``constraints`` option
on subclasses directly, providing a unique name for each constraint.
.. 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 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.
``name``
--------
.. attribute:: CheckConstraint.name
The name of the constraint.
``UniqueConstraint``
====================
.. class:: UniqueConstraint(*, fields, name, condition=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`.
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