Dubious code in multiple_database tests

[Aymeric Augustin]

QueryTestCase.test_o2o_cross_database_protection contains code that seems invalid to me.

In that test, UserProfile has a nullable one-to-one relation to User.

An unsaved instance of User called charlie is created at line 655, and a profile is attached to this instance at line 664: charlie.userprofile = bob_profile.

But after saving everything, that doesn't work as expected: the profile isn't linked to the user!

This is easy to understand: this line basically performs bob_profile.user_id = charlie.id (and populates relation caches); but since charlie is unsaved, charlie.id is None, hence bob_profile.used_id remains None.

I'm attaching a patch for this test case that reloads the objects and tests that charlie.userprofile == bob_profile ; with this patch, the test fails, proving my point.

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In #18153, I tried to catch such programming errors by raising an exception. And that breaks that test.

I think this code needs to fixed, while preserving as much of the testing behavior as possible.

[Aymeric Augustin <aymeric.augustin@…>]

In c9b4e9ac3a6c56f2b006b723972e77722d3ac937:

Fixed #19095 -- invalid code in multidb tests.