Also, according to the comments on the ticket and its duplicates, added
tests execising saving an instance of a model with a GFK to:
* An unsaved object -- This actually doesn't generate the same failure
but another ORM-level exception. The test verifies it's the case.
* An instance of a model with a __nonzero__() method thant returns False
for it. This doesn't fail because that code path isn't executed.
* An instance of a model with a CharField PK and an empty value for it.
This doesn't fail.
The ORM didn't reuse joins for direct foreign key traversals when using
chained filters. For example:
qs.filter(fk__somefield=1).filter(fk__somefield=2))
produced two joins.
As a bonus, reverse onetoone filters can now reuse joins correctly
The regression was caused by the join() method refactor in commit
68847135bc
Thanks for Simon Charette for spotting some issues with the first draft
of the patch.
