PostgreSQL specific database constraints¶
PostgreSQL supports additional data integrity constraints available from the
django.contrib.postgres.constraints
module. They are added in the model
Meta.constraints
option.
ExclusionConstraint
¶
- class ExclusionConstraint(*, name, expressions, index_type=None, condition=None, deferrable=None, include=None, violation_error_code=None, violation_error_message=None)[source]¶
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
BtreeGistExtension
migration operation.If you attempt to insert a new row that conflicts with an existing row, an
IntegrityError
is raised. Similarly, when update conflicts with an existing row.Exclusion constraints are checked during the model validation.
name
¶
- ExclusionConstraint.name¶
See BaseConstraint.name
.
expressions
¶
- 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 RangeOperators
which maps the
operators with strings. For example:
expressions = [
("timespan", RangeOperators.ADJACENT_TO),
(F("room"), RangeOperators.EQUAL),
]
Restrictions on operators.
Only commutative operators can be used in exclusion constraints.
The OpClass()
expression can
be used to specify a custom operator class for the constraint expressions.
For example:
expressions = [
(OpClass("circle", name="circle_ops"), RangeOperators.OVERLAPS),
]
creates an exclusion constraint on circle
using circle_ops
.
index_type
¶
- 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
¶
- ExclusionConstraint.condition¶
A 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
django.db.models.Index.condition
.
deferrable
¶
- 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
¶
- ExclusionConstraint.include¶
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
(include
) and filter only by indexed fields
(expressions
).
include
is supported for GiST indexes. PostgreSQL 14+ also supports
include
for SP-GiST indexes.
violation_error_code
¶
- ExclusionConstraint.violation_error_code¶
The error code used when ValidationError
is raised during
model validation. Defaults to None
.
violation_error_message
¶
The error message used when ValidationError
is raised during
model validation. Defaults to
BaseConstraint.violation_error_message
.
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
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),
),
]