# Django 3.0.9.dev - dokumentacja

# 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`

New in Django 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. 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.

### `name`

`ExclusionConstraint.``name`

The name of the constraint.

### `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.

### `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`.

### 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),
                ),
            ]