Ensure DynamicRelationField.to_representation is passed correct instance

[fdintino]

This fixes a regression that was originally introduced in d0c31d3 and a869d28

There were a few places in our code where we have subclassed DynamicRelationField and overridden to_representation, conditionally short-circuiting the serialization of a many to many field depending on the request, context, or a property of the parent instance. We had been relying on the fact that to_representation for a ModelField and a DynamicRelationField was always passed the parent instance. But in d0c31d3, and then a869d28, some of the responsibilities of to_representation were moved over to get_attribute.

After updating to the latest version our methods no longer worked, because to_representation was now being passed the ManyRelatedManager, not the parent instance. This seems to me like a bug, as it breaks expectations about the arguments and return values of get_attribute and to_representation. And so far as I can tell, moving the logic back from get_attribute doesn't break anything in dynamic-rest, since the two methods are only ever called together, e.g.: in WithDynamicSerializerMixin._faster_to_representation():

attribute = field.get_attribute(instance)
# ...
ret[field.field_name] = field.to_representation(attribute)

I've included a test that shows roughly how we sometimes override to_representation.

[fdintino]

This PR was closed accidentally. Re-opening.

[fdintino]

Any chance this might get a review and merged? We're currently installing from our github fork, but we'd prefer to avoid that if we can. Thanks!

[defbyte]

Seconding the request for eyes on this, it would be super helpful ❤️

[fdintino]

Let me know if there's anything I can to do to help get this merged in. We'd prefer it if we didn't need to maintain a fork. Thank you!

[simkimsia]

@suavesav can help?