===================== 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 ` 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.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 ` 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`.