Defer FileFields causes errors when accessing them

[Samuel Maudo]

The problem is in FileDescriptor.__get__(). That method obtains the file directly from instance.__dict__ but, from Django 1.10, deferred fields don't create any item in instance.__dict__, so a KeyError is raised.

Moreover, ImageFields access the file each time a model is instantiated to update dimension fields, so a KeyError is raised whenever an instance with a deferred ImageField is created.

The obvious solution is change file = instance.__dict__[self.field.name] for file = instance.__dict__.get(self.field.name) inside of FileDescriptor.__get__() but I am not sure about the consequences.

[Tim Graham]

A regression test for Django's test suite

[Tim Graham]

​PR

[Samuel Maudo]

The patch has a problem with ImageFields, I thought. Since image file is requiered whenever the model is instantiated, defer a ImageField results in an extra database query for each object in the QuerySet. That could generate hundreds of database queries.

[Tim Graham]

I've updated the patch.

[Tim Graham <timograham@…>]

In 7c33aa8a:

Fixed #26900 -- Fixed crash accessing deferred FileFields.

[Tim Graham <timograham@…>]

In a03ac613:

[1.10.x] Fixed #26900 -- Fixed crash accessing deferred FileFields.

Backport of 7c33aa8a87d323f0e8e5368705aa8ba96f9819d0 from master