Fixed #29824 -- Added support for database exclusion constraints on PostgreSQL.

Thanks to Nick Pope and Mariusz Felisiak for review.

Co-Authored-By: Mariusz Felisiak <felisiak.mariusz@gmail.com>

docs/ref/contrib/postgres/constraints.txt

@@ -0,0 +1,151 @@
+========================================
+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 <django.db.models.Options.constraints>` option.
+
+``ExclusionConstraint``
+=======================
+
+.. versionadded:: 3.0
+
+.. class:: ExclusionConstraint(*, name, expressions, index_type=None, condition=None)
+
+    Creates an exclusion constraint in the database. Internally, PostgreSQL
+    implements exclusion constraints using indexes. The default index type is
+    `GiST <https://www.postgresql.org/docs/current/gist.html>`_. To use them,
+    you need to activate the `btree_gist extension
+    <https://www.postgresql.org/docs/current/btree-gist.html>`_ 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 a 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`.
+
+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 <https://www.postgresql.org/docs/
+current/rangetypes.html#RANGETYPES-INCLUSIVITY>`_. 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),
+                ),
+            ]