======================================== PostgreSQL specific database constraints ======================================== .. module:: django.contrib.postgres.constraints :synopsis: PostgreSQL specific database constraint PostgreSQL supports additional data integrity constraints available from the ``django.contrib.postgres.constraints`` module. They are added in the model :attr:`Meta.constraints ` option. ``ExclusionConstraint`` ======================= .. class:: ExclusionConstraint(*, name, expressions, index_type=None, condition=None, deferrable=None, include=None, opclasses=()) Creates an exclusion constraint in the database. Internally, PostgreSQL implements exclusion constraints using indexes. The default index type is `GiST `_. To use them, you need to activate the `btree_gist extension `_ on PostgreSQL. You can install it using the :class:`~django.contrib.postgres.operations.BtreeGistExtension` migration operation. If you attempt to insert a new row that conflicts with an existing row, an :exc:`~django.db.IntegrityError` is raised. Similarly, when update conflicts with an existing row. ``name`` -------- .. attribute:: ExclusionConstraint.name The name of the constraint. ``expressions`` --------------- .. attribute:: ExclusionConstraint.expressions An iterable of 2-tuples. The first element is an expression or string. The second element is an SQL operator represented as a string. To avoid typos, you may use :class:`~django.contrib.postgres.fields.RangeOperators` which maps the operators with strings. For example:: expressions=[ ('timespan', RangeOperators.ADJACENT_TO), (F('room'), RangeOperators.EQUAL), ] .. admonition:: Restrictions on operators. Only commutative operators can be used in exclusion constraints. ``index_type`` -------------- .. attribute:: ExclusionConstraint.index_type The index type of the constraint. Accepted values are ``GIST`` or ``SPGIST``. Matching is case insensitive. If not provided, the default index type is ``GIST``. ``condition`` ------------- .. attribute:: ExclusionConstraint.condition A :class:`~django.db.models.Q` object that specifies the condition to restrict a constraint to a subset of rows. For example, ``condition=Q(cancelled=False)``. These conditions have the same database restrictions as :attr:`django.db.models.Index.condition`. ``deferrable`` -------------- .. attribute:: ExclusionConstraint.deferrable Set this parameter to create a deferrable exclusion constraint. Accepted values are ``Deferrable.DEFERRED`` or ``Deferrable.IMMEDIATE``. For example:: from django.contrib.postgres.constraints import ExclusionConstraint from django.contrib.postgres.fields import RangeOperators from django.db.models import Deferrable ExclusionConstraint( name='exclude_overlapping_deferred', expressions=[ ('timespan', RangeOperators.OVERLAPS), ], 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. .. warning:: Deferred exclusion constraints may lead to a `performance penalty `_. ``include`` ----------- .. attribute:: ExclusionConstraint.include .. versionadded:: 3.2 A list or tuple of the names of the fields to be included in the covering exclusion constraint as non-key columns. This allows index-only scans to be used for queries that select only included fields (:attr:`~ExclusionConstraint.include`) and filter only by indexed fields (:attr:`~ExclusionConstraint.expressions`). ``include`` is supported only for GiST indexes on PostgreSQL 12+. ``opclasses`` ------------- .. attribute:: ExclusionConstraint.opclasses .. versionadded:: 3.2 The names of the `PostgreSQL operator classes `_ to use for this constraint. If you require a custom operator class, you must provide one for each expression in the constraint. For example:: ExclusionConstraint( name='exclude_overlapping_opclasses', expressions=[('circle', RangeOperators.OVERLAPS)], opclasses=['circle_ops'], ) creates an exclusion constraint on ``circle`` using ``circle_ops``. Examples -------- The following example restricts overlapping reservations in the same room, not taking canceled reservations into account:: from django.contrib.postgres.constraints import ExclusionConstraint from django.contrib.postgres.fields import DateTimeRangeField, RangeOperators from django.db import models from django.db.models import Q class Room(models.Model): number = models.IntegerField() class Reservation(models.Model): room = models.ForeignKey('Room', on_delete=models.CASCADE) timespan = DateTimeRangeField() cancelled = models.BooleanField(default=False) class Meta: constraints = [ ExclusionConstraint( name='exclude_overlapping_reservations', expressions=[ ('timespan', RangeOperators.OVERLAPS), ('room', RangeOperators.EQUAL), ], condition=Q(cancelled=False), ), ] In case your model defines a range using two fields, instead of the native PostgreSQL range types, you should write an expression that uses the equivalent function (e.g. ``TsTzRange()``), and use the delimiters for the field. Most often, the delimiters will be ``'[)'``, meaning that the lower bound is inclusive and the upper bound is exclusive. You may use the :class:`~django.contrib.postgres.fields.RangeBoundary` that provides an expression mapping for the `range boundaries `_. For example:: from django.contrib.postgres.constraints import ExclusionConstraint from django.contrib.postgres.fields import ( DateTimeRangeField, RangeBoundary, RangeOperators, ) from django.db import models from django.db.models import Func, Q class TsTzRange(Func): function = 'TSTZRANGE' output_field = DateTimeRangeField() class Reservation(models.Model): room = models.ForeignKey('Room', on_delete=models.CASCADE) start = models.DateTimeField() end = models.DateTimeField() cancelled = models.BooleanField(default=False) class Meta: constraints = [ ExclusionConstraint( name='exclude_overlapping_reservations', expressions=( (TsTzRange('start', 'end', RangeBoundary()), RangeOperators.OVERLAPS), ('room', RangeOperators.EQUAL), ), condition=Q(cancelled=False), ), ]