# Τεκμηρίωση του Django - 5.2.8

# 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)

  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.

Προειδοποίηση

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),
                ),
            ]