Relation fields no to_fields attribute attribute break serializer

[martinhill]

Hello,

I have a ModelSerializer for the django-taggit package's Tag model. With DRF release 3.3.1 it began failing with AttributeError: 'TaggableManager' object has no attribute 'to_fields'.

I believe this line from PR 3526 assumes all relation fields have a to_fields attribute, but taggit's TaggableManager is an example of one that doesn't.

Replacing it with

    return field.to_fields[0] if hasattr(field, 'to_fields') else None

seems to solve the problem.

[xordoquy]

Seems sensible to me.

[jpadilla]

@martinhill wan't to put a PR together with your proposed changes?

[martinhill]

be happy to!