Deferred fields not passed to inherited models' __class__.__dict__

[Jarek Glowacki]

Possibly a regression, possibly me misunderstanding how this should now work in 1.10.

class A(models.Model):
    name = models.CharField(max_length=20)
    addy = models.CharField(max_length=200)


class B(A):
    name2 = models.CharField(max_length=20)

Calling:

b = B.objects.only('name').first()

we expect name2 and addy to get deferred. Calling b.get_deferred_fields(), we get both these field names returned as expected.

However, inspecting b.__class__.__dict__, we see that only the DeferredAttribute for name2 is available. To get the one for addy, we need to consult b's parent. This was not the case in 1.9.

See my ​PR to django-model-utils for context.

[Tim Graham]

I'd guess 7f51876f99851fdc3fef63aecdfbcffa199c26b9 is the commit in question. Can you come up with a test case that demonstrates a bug in normal Django usage? If not, that change sounds like an implementation detail. We might mention it in the release notes if you feel it might affect other projects.

[Jarek Glowacki]

Thanks, I've verified it's definitely commit 7f51876f99851fdc3fef63aecdfbcffa199c26b9 that causes this change in behaviour.

DeferredAttribute instances inherited from parent models can no longer be obtained through instance.__class__.__dict__ because the class is ​no longer dynamically created, but rather has the DeferredAttribute instances ​stapled on with setattr.

However, thanks to ​this change, a handle to the DeferredAttribute descriptor can be obtained by accessing the field directly from the instance's class, which looks considerably nicer and does pull through inherited fields.

So
deferred_field = instance.__class__.__dict__.get(field_name) # pre dj110
becomes
deferred_field = getattr(instance.__class__, field_name) # dj110+

I guess it can't hurt mentioning this in the release notes, though at the same time I don't see particularly many projects that would need to handle the DeferredAttribute descriptors directly. You're in a better position to make that call. At the very least this ticket can serve as an answer to people who google this problem.

Closing.