Need warning when UniqueConstraint with condition is used but supports_partial_indexes is False

[Matthijs Kooijman]

Databases that support supports_partial_indexes allow using a UniqueConstraint with a condition specified (implemented in #29547 and #30062). However, when such constraints are specified, but the database (e.g. Mysql/Mariadb or older versions of sqlite) does not support partial indexes, these constraints are silently ignored.

It would be better if this would give a warning, similar to the warning for CheckConstraints at ​https://github.com/django/django/blob/9358da704ea9ba55f22df912e47b54eb85d5c97e/django/db/models/base.py#L1838-L1862

I have observed this with 2.2.10, but looking at the code, this has not changed since then, so I'm setting version to master.

This is probably a fairly trivial change, but I'm not in the opportunity to provide a PR in the near future.

[Ichlasul Affan]

As mentioned by #30062, the warning should be given when using MySQL/MariaDB or SQLite version <= 3.8.

[Ichlasul Affan]

Replying to Matthijs Kooijman:

Databases that support supports_partial_indexes allow using a UniqueConstraint with a condition specified (implemented in #29547 and #30062). However, when such constraints are specified, but the database (e.g. Mysql/Mariadb or older versions of sqlite) does not support partial indexes, these constraints are silently ignored.

It would be better if this would give a warning, similar to the warning for CheckConstraints at ​https://github.com/django/django/blob/9358da704ea9ba55f22df912e47b54eb85d5c97e/django/db/models/base.py#L1838-L1862

I have observed this with 2.2.10, but looking at the code, this has not changed since then, so I'm setting version to master.

This is probably a fairly trivial change, but I'm not in the opportunity to provide a PR in the near future.

Firstly, I'm sorry I am new contributor to Django. When I explored further on the code, the warning checks for specific DBMS' ability to check constraints using connection.features.support_table_check_constraints. Unfortunately, I think that similiar type of variable would be available on some database connectors for this case (unique constraint with conditions). Would you mind if I just add the warning regardless of what kind of database users will use?

UPDATE: Oh nevermind, I can just use support_partial_indexes. The default definitions is actually available via django.db.backends.base.features.

[Matthijs Kooijman]

Yeah, I think the check can just look at supports_partial_indexes, since that is also what [the code that decides (not) to add the constraint](​https://github.com/django/django/blob/57a3d96ff57abffa8e7fbe3fd972a5ee39bd0fec/django/db/backends/base/schema.py#L1097) looks at.

[Ichlasul Affan]

PR: ​https://github.com/django/django/pull/12559

[Ian Foote]

I just had a look over the PR and it looks like the outstanding issues have been resolved.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 53d229ff:

Fixed #31351 -- Added system checks for partial indexes and unique constraints support.

[GitHub <noreply@…>]

In 4c988608:

Refs #31351 -- Made partial constraints tests use required_db_features.

This will notably silence the warnings issued when running the test
suite on MySQL and MariaDB.