Fix unchanging field bug when validate assignment true

[austinpgraham]

Problem

This PR addresses a bug reported here, where if validate_assignment is set to True on a schema, then Pydantic will correctly validate the new values, but the schema object will not persist the new value.

RCA

This is caused by the fact that pydantic-core sets these new values on the object (once validated) directly on the object __ dict __. As it was, this means that the new value will be set onto the DjangoGetter object, rather than the schema object being validated.

Solution

Added an extra condition in the root validator of a Schema object that will call handler on the original values before the conversion to a DjangoGetter to ensure the new value is persisted properly on the original object. Comment is also updated to reflect this change.

Testing

Added assertions into an existing test that a reassigned value is persisted when validate_assignment = True, and an additional test to test for the lack of ValidationError when validate_assignment is False or None.

Open Questions

This seems odd in the following way: I understand the use of DjangoGetter of course to handle resolvable fields. However, it not behaving like a dictionary is causing some churn where we now have to validate the object twice in certain conditions just to ensure fields are persisted/validated properly. I don't know immediately if there's a better solution, or if there's background here I don't know. Welcome to feedback.

[austinpgraham]

@vitalik potential fix to issue mentioned here: #1223

[vitalik]

Thank you