fields.get_attribute: return None for FK lookups

[blueyed]

Ref: #5708 (comment)

TODO:

• test

[blueyed]

"lint" CI failure is unrelated.

Some advice for where to put a test would be nice, if this change makes sense in general.
(no current tests are failing)

[rpkilby]

Adding this to my todo list. In general, I need to go back and check the behavior around missing FK's, similar to the changes made in #5375.

[carltongibson]

Hey @blueyed. Sorry for the slow follow-up here.

Some advice for where to put a test would be nice...

I don't mind where so much to begin — perhaps just drop something into test fields — but a reproduce in a test case in this PR would be great. Any chance you can whip one up?

Thanks!

[carltongibson]

OK… I added a test covering the problem use-case from #5375 (in #5880). I've rebased this on that.
It'll now fail.

From #5375 (comment)

Without the guard, a subsequent attr would cause an AttributeError to be raised instead, allowing the default to be used.

In order to support correct default behaviour with dotted sources where any path element may be None we must raise the AttributeError in rest_framework.fields.get_attribute.

Once the build completes, I will close this one as-is. It looks like a non-starter. (Happy to reopen, or do it on a new PR, if we can make progress.)

I'm still looking at the underlying issue.