# Τεκμηρίωση του Django - 3.2.11.dev

# Constraints reference

The classes defined in this module create database constraints. They are added in the model `Meta.constraints` option.

Referencing built-in constraints

Constraints are defined in `django.db.models.constraints`, but for convenience they’re imported into `django.db.models`. The standard convention is to use `from django.db import models` and refer to the constraints as `models.<Foo>Constraint`.

Constraints in abstract base classes

You must always specify a unique name for the constraint. As such, you cannot normally specify a constraint on an abstract base class, since the `Meta.constraints` option is inherited by subclasses, with exactly the same values for the attributes (including `name`) each time. To work around name collisions, part of the name may contain `'%(app_label)s'` and `'%(class)s'`, which are replaced, respectively, by the lowercased app label and class name of the concrete model. For example `CheckConstraint(check=Q(age__gte=18), name='%(app_label)s_%(class)s_is_adult')`.

Validation of Constraints

In general constraints are not checked during `full_clean()`, and do not raise `ValidationError`s. Rather you’ll get a database integrity error on `save()`. `UniqueConstraint`s without a `condition` (i.e. non-partial unique constraints) are different in this regard, in that they leverage the existing `validate_unique()` logic, and thus enable two-stage validation. In addition to `IntegrityError` on `save()`, `ValidationError` is also raised during model validation when the `UniqueConstraint` is violated.

## `CheckConstraint`

class `CheckConstraint`(*, check, name)

  Creates a check constraint in the database.

### `check`

`CheckConstraint.``check`

A `Q` object or boolean `Expression` that specifies the check you want the constraint to enforce.

For example, `CheckConstraint(check=Q(age__gte=18), name='age_gte_18')` ensures the age field is never less than 18.

Changed in Django 3.1:

Support for boolean `Expression` was added.

### `name`

`CheckConstraint.``name`

The name of the constraint. You must always specify a unique name for the constraint.

## `UniqueConstraint`

class `UniqueConstraint`(*, fields, name, condition=None, deferrable=None, include=None, opclasses=())

  Creates a unique constraint in the database.

### `fields`

`UniqueConstraint.``fields`

A list of field names that specifies the unique set of columns you want the constraint to enforce.

For example, `UniqueConstraint(fields=['room', 'date'], name='unique_booking')` ensures each room can only be booked once for each date.

### `name`

`UniqueConstraint.``name`

The name of the constraint. You must always specify a unique name for the constraint.

### `condition`

`UniqueConstraint.``condition`

A `Q` object that specifies the condition you want the constraint to enforce.

For example:

    UniqueConstraint(fields=['user'], condition=Q(status='DRAFT'), name='unique_draft_user')

ensures that each user only has one draft.

These conditions have the same database restrictions as `Index.condition`.

### `deferrable`

`UniqueConstraint.``deferrable`

New in Django 3.1.

Set this parameter to create a deferrable unique constraint. Accepted values are `Deferrable.DEFERRED` or `Deferrable.IMMEDIATE`. For example:

    from django.db.models import Deferrable, UniqueConstraint

    UniqueConstraint(
        name='unique_order',
        fields=['order'],
        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.

MySQL, MariaDB, and SQLite.

Deferrable unique constraints are ignored on MySQL, MariaDB, and SQLite as neither supports them.

Προειδοποίηση

Deferred unique constraints may lead to a performance penalty.

### `include`

`UniqueConstraint.``include`

New in Django 3.2.

A list or tuple of the names of the fields to be included in the covering unique index as non-key columns. This allows index-only scans to be used for queries that select only included fields (`include`) and filter only by unique fields (`fields`).

For example:

    UniqueConstraint(name='unique_booking', fields=['room', 'date'], include=['full_name'])

will allow filtering on `room` and `date`, also selecting `full_name`, while fetching data only from the index.

`include` is supported only on PostgreSQL.

Non-key columns have the same database restrictions as `Index.include`.

### `opclasses`

`UniqueConstraint.``opclasses`

New in Django 3.2.

The names of the PostgreSQL operator classes to use for this unique index. If you require a custom operator class, you must provide one for each field in the index.

For example:

    UniqueConstraint(name='unique_username', fields=['username'], opclasses=['varchar_pattern_ops'])

creates a unique index on `username` using `varchar_pattern_ops`.

`opclasses` are ignored for databases besides PostgreSQL.