refresh_from_db leaves FieldFile with reference to db_instance

[vinnyrose]

#### models.py

class Product(models.Model):
    myfile = models.FileField(upload_to='folder')

##### elsewhere.py

product = Product.objects.create(myfile='filename')
product.refresh_from_db()

assert id(product.myfile.instance) == id(product) # false

product.file.instance is actually db_instance which is used internally in refresh_from_db.

This leads to strange results when trying to use FieldFile.save or FieldFile.delete, since you would be modifying db_instance instead of the intended instance. This also means any field that uses a descriptor_class that returns an attr_class will have this same issue, I've used this pattern for custom fields.

This could most simply be fixed in the FileDescriptor itself, because frankly I don't see how this could be fixed for all cases in which a field has reference to the instance from which it was instatianted from. Perhaps a warning on refresh_from_db in the docs about this risk?

[Attila Tovt]

​PR

[Tim Graham <timograham@…>]

In 6f229048:

Fixed #25547 -- Made Model.refresh_from_db() update FileField's instance.