Abstract models containing ImageSpecFields are not handled properly

[seanbell]

One of my models is abstract and includes several ImageSpecField fields. Calling ./manage.py generateimages leads to an exception in imagekit/specs/sourcegroups.py at the line:

    for instance in self.model_class.objects.all():

The attached commit fixes this particular exception, but I don't think it addresses all the potential issues involved with abstract base classes. For example, the generator registry only includes one entry corresponding to the abstract base; it may make sense to instead list it for all the descendants that correspond to non-abstract models.

[matthewwithanm]

Thanks @seanbell! I think you found an issue not only with abstract models, but also model subclasses in general. However, I think your get_nonabstract_descendants() didn't quite solve it since it only returned the first non-abstract descendants, and not their descendants. (I added a test to illustrate.)

As for your concern about the generator registry, all the handling of sources is done by the source groups, so I don't think we have any problems there. The source group just needs to be aware of the subclasses. I think we should be good with the commits I just pushed.

I've bumped the version to 3.0a3. Thanks so much for the report and the PR! Let me know if it works for you now.

[seanbell]

Great! It seems to work for me.

[matthewwithanm]

Fantastic! Thanks again for your help!