Incorrect usage of constraint_checks_disabled in tests

[wlodek]

If a backend implements enable/disable_constraint_checking (using database SET CONSTRAINTS) some of the tests fail.

The problem comes from the fact that the usage of context manager "constraint_checks_disabled" is based not on the assumptions about how it should work but on the particular implementation for core backends ie. that constraints are checked at transaction commit.

Because enable/disable_constraint_checking for those backends are empty constraint_checks_disabled context manager does nothing and the tests pass while in fact they should fail.

As I understand "assumptions about how it should work" are such that context manager constraint_checks_disabled is for constraints checks something like savepoints for transactions, ie. you can mark a set of statements inside a transaction such the constraints could not be met inside that set but should at the end of set/exit from context manager.

Affected are tests in 'backends', 'serializers_regress', ...

For example in test_disable_constraint_checks_context_manager save inside context manager violates constraints IntegrityError should be raised, it we want to check that it isn't raised we should correct the references before context manager exit.

try:
    with connection.constraint_checks_disabled():
        a.save()
        a.reporter_id= self.r.id
        a.save()
except IntegrityError:
    self.fail("IntegrityError should not have occurred.")

Beside that there is a question why enable/disable_constraint_checking are empty for core backends that could implement that like oracle/postgres? The fact that those backends make use of INITIALY DEFERRED constraints doesn't mean that those must be empty and so test_disable_constraint_checks_context_manager unusable.

[ANUBHAV JOSHI]

How can I reproduce this?

[Aymeric Augustin]

Indeed, constraint_checks_disabled and related APIs exist only because MySQL is unable to defer constraints checks until transaction commit. See #3615 for details.

I read your description a few times and I don't understand what your proposal actually is. Maybe you could take this to the DevelopersMailingList?

[Shai Berger]

This actually started on the ​mailing list.