possibly unneccessary unique_check for primary key fields is models

[Mark]

Every time a clean() method of the model instance is run, validate_unique() is performed. It seems, that _get_unique_checks doesn't get into account, that the primary key doesn't need such a check. The sql that is run during the check is meaningless (it is only meaningless if we are checking a primary key field):

SELECT (1) AS "a" FROM "test_models_testmodel" WHERE ("test_models_testmodel"."id" = 1  AND NOT ("test_models_testmodel"."id" = 1 ))

This results in unnecessary database hits and (possible) performance degradation. I have put together a simple test rack, so that the effect can be easily observed. It's not a real test case, but I really hope, that it will be sufficient.

If I understand correctly, it can be easily fixed, by checking if a field is a primary key inside the _get_unique_checks model instance method (patch included).

[Mark]

A runnable test and an app directory

[Mark]

Proposed fix

[Honza Král]

(In [11856]) [soc2009/model-validation] Fixed #12132: unneccessary unique_check for primary key when not adding model