Set up serializer fields lazily on-demand.

[carljm]

This avoids AppRegistryNotReady problems in Django 1.7 with nested serializers,
which are instantiated at import time, possibly before Django's app registry is
fully populated.

Fixes #1907 and #1943.

[carljm]

The "test" here is a bit ugly, since it's hard to effectively test these global-state-setup issues. I'm importing and instantiating a model serializer (with fields) in the pytest_configure hook, before django.setup() is called. Without the fix, this causes the entire test run to blow up :/

I'm not sure how else to effectively test this, but I'll happily remove that "test" if it's considered too ugly.

[xordoquy]

Thanks a lot. This will definitively help developers porting to Django 1.7.
At first I was a bit afraid it would make the fields attribute impossible to alter but after a look at cached_property this should not be an issue at all.
It looks fine to me. I'm leaving it opened to get another review.

[carljm]

@xordoquy Yes, cached_property is really quite simple and clever.

[carljm]

Oh, I should explicitly note too that I had to modify a couple other tests asserting that field-configuration errors would be raised on model instantiation, to instead assert that those errors would be raised on first access of the fields property.

[carljm]

One other note - the app-registry errors may still bite anyone who accesses self.fields in an overridden serializer __init__ method, and then uses that serializer as a nested field. But at least with this fix they should be able to find a different approach that avoids the problem.

[carljm]

@xordoquy I'm not sure why this is labeled an Enhancement when #1907 and #1943, which it fixes, are both labeled Bugs?

[tomchristie]

I think I'd rather just drop this. The hackery required to hard-test the behaviour doesn't seem worth the trade-off.

[carljm]

Ok, force-pushed a new single commit without the test hackery.

[tomchristie]

Looks like we'd need a bit of minor tweaking of 3.0 to support this, but nothing problematic.

[tomchristie]

Marginally more involved in 3.0 to do this, but resolved there now.