# Django 2.2.9.dev20191202104323 文档

# Constraints reference

New in Django 2.2.

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. Instead, specify the `constraints` option on subclasses directly, providing a unique name for each constraint.

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 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.

### `name`

`CheckConstraint.``name`

The name of the constraint.

## `UniqueConstraint`

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

  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.

### `condition`

`UniqueConstraint.``condition`

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

例子:

    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`.