Check that the foreign object `from_fields` are a subset of any unique
constraints on the foreign model.
Backport of 80dac8c33e7f6f22577e4346f44e4c5ee89b648c and
c7aff31397a7228f6ac2e33c10ebdf36c4b7a9b7 from master
This also updates all dependent functionality, including modelform_factory
and modelformset_factory, and the generic views `ModelFormMixin`,
`CreateView` and `UpdateView` which gain a new `fields` attribute.
This patch iproved two major parts in Django. First, the fields.related
was refactored. The main addition there was ForeignObject. Second, the
ORM now handles multicolumn joins in most cases, though there are still
cases that do not work correcly (split_exclude() for example).
In addition there were extesive changes to how GenericRelation works.
Before it was a fake m2m field, now it is a pure virtual fields and is
based on ForeignObject.
There is still much room for improvement. The related fields code is
still somewhat confusing, and how fields are represented in model._meta
should also be revisited.
This patch was written mostly by Jeremy Tillman with some final polish
by the committer.
