Document what happens during model validation with Meta.constraints.

[Dirk Groten]

The current ​constraints reference does not mention whether or not a ValidationError is raised during model validation if a constraint fails. The documentation only mentions that a database constraint is created.

However, for the soon to be deprecated unique_together, there is a reference to model validation.

As far as I can see, a UniqueConstraint causes validate_unique() to check for uniqueness and raises a ValidationError. But a CheckConstraint does not cause a ValidationError when the model is validated. I believe it's important to mention this in the documentation.

[Mariusz Felisiak]

​PR

[Carlton Gibson <carlton.gibson@…>]

In 00169bc3:

Fixed #30547 -- Doc'd how Meta.constraints affect model validation.

[Carlton Gibson <carlton.gibson@…>]

In c3a0f76d:

[2.2.x] Fixed #30547 -- Doc'd how Meta.constraints affect model validation.

Backport of 00169bc36156d4a32546229bf39de91213709eaf from master

[Kevin Christopher Henry]

I suggest that the note be expanded to indicate that uniqueness validation does not work if the condition parameter is used (i.e. for partial unique indexes). Based on ​this comment it appears to be a hard problem, and unlikely to be implemented any time soon.

[Mariusz Felisiak]

Good catch! I added clarification in ​PR.

[GitHub <noreply@…>]

In 230d75f5:

Refs #30547 -- Clarified that partial UniqueConstraints don't affect model validation.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 0088e592:

[2.2.x] Refs #30547 -- Clarified that partial UniqueConstraints don't affect model validation.

Backport of 230d75f59c43b9731465c4ec92ad714e301637b8 from master