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230 lines
7.7 KiB
Plaintext
230 lines
7.7 KiB
Plaintext
========================================
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PostgreSQL specific database constraints
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========================================
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.. module:: django.contrib.postgres.constraints
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:synopsis: PostgreSQL specific database constraint
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PostgreSQL supports additional data integrity constraints available from the
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``django.contrib.postgres.constraints`` module. They are added in the model
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:attr:`Meta.constraints <django.db.models.Options.constraints>` option.
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``ExclusionConstraint``
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=======================
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.. class:: ExclusionConstraint(*, name, expressions, index_type=None, condition=None, deferrable=None, include=None, violation_error_code=None, violation_error_message=None)
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Creates an exclusion constraint in the database. Internally, PostgreSQL
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implements exclusion constraints using indexes. The default index type is
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`GiST <https://www.postgresql.org/docs/current/gist.html>`_. To use them,
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you need to activate the `btree_gist extension
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<https://www.postgresql.org/docs/current/btree-gist.html>`_ on PostgreSQL.
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You can install it using the
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:class:`~django.contrib.postgres.operations.BtreeGistExtension` migration
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operation.
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If you attempt to insert a new row that conflicts with an existing row, an
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:exc:`~django.db.IntegrityError` is raised. Similarly, when update
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conflicts with an existing row.
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Exclusion constraints are checked during the :ref:`model validation
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<validating-objects>`.
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``name``
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--------
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.. attribute:: ExclusionConstraint.name
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See :attr:`.BaseConstraint.name`.
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``expressions``
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---------------
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.. attribute:: ExclusionConstraint.expressions
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An iterable of 2-tuples. The first element is an expression or string. The
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second element is an SQL operator represented as a string. To avoid typos, you
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may use :class:`~django.contrib.postgres.fields.RangeOperators` which maps the
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operators with strings. For example::
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expressions = [
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("timespan", RangeOperators.ADJACENT_TO),
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(F("room"), RangeOperators.EQUAL),
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]
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.. admonition:: Restrictions on operators.
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Only commutative operators can be used in exclusion constraints.
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The :class:`OpClass() <django.contrib.postgres.indexes.OpClass>` expression can
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be used to specify a custom `operator class`_ for the constraint expressions.
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For example::
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expressions = [
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(OpClass("circle", name="circle_ops"), RangeOperators.OVERLAPS),
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]
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creates an exclusion constraint on ``circle`` using ``circle_ops``.
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.. _operator class: https://www.postgresql.org/docs/current/indexes-opclass.html
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``index_type``
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--------------
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.. attribute:: ExclusionConstraint.index_type
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The index type of the constraint. Accepted values are ``GIST`` or ``SPGIST``.
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Matching is case insensitive. If not provided, the default index type is
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``GIST``.
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``condition``
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-------------
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.. attribute:: ExclusionConstraint.condition
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A :class:`~django.db.models.Q` object that specifies the condition to restrict
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a constraint to a subset of rows. For example,
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``condition=Q(cancelled=False)``.
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These conditions have the same database restrictions as
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:attr:`django.db.models.Index.condition`.
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``deferrable``
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--------------
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.. attribute:: ExclusionConstraint.deferrable
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Set this parameter to create a deferrable exclusion constraint. Accepted values
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are ``Deferrable.DEFERRED`` or ``Deferrable.IMMEDIATE``. For example::
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from django.contrib.postgres.constraints import ExclusionConstraint
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from django.contrib.postgres.fields import RangeOperators
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from django.db.models import Deferrable
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ExclusionConstraint(
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name="exclude_overlapping_deferred",
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expressions=[
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("timespan", RangeOperators.OVERLAPS),
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],
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deferrable=Deferrable.DEFERRED,
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)
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By default constraints are not deferred. A deferred constraint will not be
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enforced until the end of the transaction. An immediate constraint will be
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enforced immediately after every command.
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.. warning::
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Deferred exclusion constraints may lead to a `performance penalty
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<https://www.postgresql.org/docs/current/sql-createtable.html#id-1.9.3.85.9.4>`_.
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``include``
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-----------
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.. attribute:: ExclusionConstraint.include
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A list or tuple of the names of the fields to be included in the covering
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exclusion constraint as non-key columns. This allows index-only scans to be
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used for queries that select only included fields
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(:attr:`~ExclusionConstraint.include`) and filter only by indexed fields
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(:attr:`~ExclusionConstraint.expressions`).
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``include`` is supported for GiST indexes. PostgreSQL 14+ also supports
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``include`` for SP-GiST indexes.
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``violation_error_code``
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------------------------
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.. versionadded:: 5.0
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.. attribute:: ExclusionConstraint.violation_error_code
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The error code used when ``ValidationError`` is raised during
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:ref:`model validation <validating-objects>`. Defaults to ``None``.
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``violation_error_message``
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---------------------------
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The error message used when ``ValidationError`` is raised during
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:ref:`model validation <validating-objects>`. Defaults to
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:attr:`.BaseConstraint.violation_error_message`.
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Examples
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--------
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The following example restricts overlapping reservations in the same room, not
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taking canceled reservations into account::
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from django.contrib.postgres.constraints import ExclusionConstraint
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from django.contrib.postgres.fields import DateTimeRangeField, RangeOperators
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from django.db import models
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from django.db.models import Q
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class Room(models.Model):
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number = models.IntegerField()
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class Reservation(models.Model):
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room = models.ForeignKey("Room", on_delete=models.CASCADE)
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timespan = DateTimeRangeField()
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cancelled = models.BooleanField(default=False)
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class Meta:
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constraints = [
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ExclusionConstraint(
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name="exclude_overlapping_reservations",
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expressions=[
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("timespan", RangeOperators.OVERLAPS),
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("room", RangeOperators.EQUAL),
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],
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condition=Q(cancelled=False),
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),
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]
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In case your model defines a range using two fields, instead of the native
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PostgreSQL range types, you should write an expression that uses the equivalent
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function (e.g. ``TsTzRange()``), and use the delimiters for the field. Most
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often, the delimiters will be ``'[)'``, meaning that the lower bound is
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inclusive and the upper bound is exclusive. You may use the
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:class:`~django.contrib.postgres.fields.RangeBoundary` that provides an
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expression mapping for the `range boundaries <https://www.postgresql.org/docs/
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current/rangetypes.html#RANGETYPES-INCLUSIVITY>`_. For example::
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from django.contrib.postgres.constraints import ExclusionConstraint
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from django.contrib.postgres.fields import (
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DateTimeRangeField,
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RangeBoundary,
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RangeOperators,
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)
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from django.db import models
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from django.db.models import Func, Q
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class TsTzRange(Func):
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function = "TSTZRANGE"
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output_field = DateTimeRangeField()
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class Reservation(models.Model):
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room = models.ForeignKey("Room", on_delete=models.CASCADE)
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start = models.DateTimeField()
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end = models.DateTimeField()
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cancelled = models.BooleanField(default=False)
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class Meta:
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constraints = [
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ExclusionConstraint(
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name="exclude_overlapping_reservations",
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expressions=[
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(
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TsTzRange("start", "end", RangeBoundary()),
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RangeOperators.OVERLAPS,
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),
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("room", RangeOperators.EQUAL),
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],
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condition=Q(cancelled=False),
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),
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]
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