refresh_from_db() doesn't clear reverse OneToOneFields

[Keith Hostetler]

Sorry for the poor summary, it is difficult to explain in words. I have a project to demo this bug attached to this ticket, but I will try to explain the bug anyway in steps and the setup.

Setup:

• 2 models (A and B)
• B has a OneToOne to A
• Both A and B have a field (ie TextField)
• Setup either a signal or override save() for A to update B's TextField to equal that of A's on save() or post_save for signals

Steps:

• Create instance of A
• Get another copy of the instance of A via A.objects.get()
• Create instance of B using the copy of the instance of A
• Do refresh_from_db() on original instance of A
• Try to access B from A

The project I have provided is a slim version of this problem that demonstrates it with signals, overriden save(), and basic set and save inside the test. The basic set and save works, but the other two fail when using the above steps. Run the test suite to see.

[Tim Graham]

I think this is fixable but I'm not sure without trying to write a patch. This case may be discussed in ticket:901#comment:36 It looks like the issue is that refresh_from_db() ​clears concrete fields but doesn't clear reverse relations.

[Paulo]

Hello,
What do you all think about clearing the fields cache entirely on refresh_from_db()?
This would cover similar bugs where a many to one or many to many relationship is cached.
The only case we can't cover is fields prefetched to custom attributes.

[Paulo]

PR: ​https://github.com/django/django/pull/9112

[Tim Graham <timograham@…>]

In a7b5ad8b:

Fixed #27846 -- Made Model.refresh_from_db() clear cached relations.

[Tim Graham <timograham@…>]

In 5d3f2aa:

[2.0.x] Fixed #27846 -- Made Model.refresh_from_db() clear cached relations.

Backport of a7b5ad8b19a08d7d57302ece74f6e26d2887fd9f from master