查看“Django/docs/3.0.x/ref/contrib/postgres/constraints”的源代码

<div id="module-django.contrib.postgres.constraints" class="section">

<span id="postgresql-specific-database-constraints"></span>
= PostgreSQL specific database constraints =

PostgreSQL supports additional data integrity constraints available from the
<code>django.contrib.postgres.constraints</code> module. They are added in the model
[[../../../models/options#django.db.models.Options|<code>Meta.constraints</code>]] option.

<div id="exclusionconstraint" class="section">

== <code>ExclusionConstraint</code> ==

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<dl>
<dt>''class'' <code>ExclusionConstraint</code><span class="sig-paren">(</span>''<span class="o">*</span>'', ''<span class="n">name</span>'', ''<span class="n">expressions</span>'', ''<span class="n">index_type</span><span class="o">=</span><span class="default_value">None</span>'', ''<span class="n">condition</span><span class="o">=</span><span class="default_value">None</span>''<span class="sig-paren">)</span></dt>
<dd><p>Creates an exclusion constraint in the database. Internally, PostgreSQL
implements exclusion constraints using indexes. The default index type is
[https://www.postgresql.org/docs/current/gist.html GiST]. To use them,
you need to activate the [https://www.postgresql.org/docs/current/btree-gist.html btree_gist extension] on PostgreSQL.
You can install it using the
[[../operations#django.contrib.postgres.operations|<code>BtreeGistExtension</code>]] migration
operation.</p>
<p>If you attempt to insert a new row that conflicts with an existing row, an
[[../../../exceptions#django.db|<code>IntegrityError</code>]] is raised. Similarly, when update
conflicts with an existing row.</p></dd></dl>

<div id="name" class="section">

=== <code>name</code> ===

; <code>ExclusionConstraint.</code><code>name</code>
: 

The name of the constraint.


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<div id="expressions" class="section">

=== <code>expressions</code> ===

; <code>ExclusionConstraint.</code><code>expressions</code>
: 

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 [[../fields#django.contrib.postgres.fields|<code>RangeOperators</code>]] which maps the
operators with strings. For example:

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<pre>expressions=[
    ('timespan', RangeOperators.ADJACENT_TO),
    (F('room'), RangeOperators.EQUAL),
]</pre>

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</div>
<div class="admonition-restrictions-on-operators admonition">

Restrictions on operators.

Only commutative operators can be used in exclusion constraints.


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<div id="index-type" class="section">

=== <code>index_type</code> ===

; <code>ExclusionConstraint.</code><code>index_type</code>
: 

The index type of the constraint. Accepted values are <code>GIST</code> or <code>SPGIST</code>.
Matching is case insensitive. If not provided, the default index type is
<code>GIST</code>.


</div>
<div id="condition" class="section">

=== <code>condition</code> ===

; <code>ExclusionConstraint.</code><code>condition</code>
: 

A [[../../../models/querysets#django.db.models|<code>Q</code>]] object that specifies the condition to restrict
a constraint to a subset of rows. For example,
<code>condition=Q(cancelled=False)</code>.

These conditions have the same database restrictions as
[[../../../models/indexes#django.db.models.Index|<code>django.db.models.Index.condition</code>]].


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<div id="examples" class="section">

=== 示例 ===

The following example restricts overlapping reservations in the same room, not
taking canceled reservations into account:

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<pre>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),
            ),
        ]</pre>

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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. <code>TsTzRange()</code>), and use the delimiters for the field. Most
often, the delimiters will be <code>'[)'</code>, meaning that the lower bound is
inclusive and the upper bound is exclusive. You may use the
[[../fields#django.contrib.postgres.fields|<code>RangeBoundary</code>]] that provides an
expression mapping for the [https://www.postgresql.org/docs/current/rangetypes.html#RANGETYPES-INCLUSIVITY range boundaries]. For example:

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<pre>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),
            ),
        ]</pre>

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