Having it happen at the lookup creation time ensures entry points
called before the compilation phase (e.g. get_group_by_cols) don't have
to duplicate the logic in charge of altering Query instances used as
rhs.
It also has the nice effect of reducing the amount of time the
alteration logic to once as opposed to multiple times if the queryset
is compiled more than once.
PathInfo values are ostensibly static over the lifetime of the object
for which they're requested, so the data can be memoized, quickly
amortising the cost over the process' duration.
Expressions should never be prepared as other Lookup.get_prep_lookup
implementations hint at by returning early on the presence of the
resolve_expression attribute.
The previous solution was only handling lookups against related fields
pointing at AutoFields and would break for foreign keys to other fields.
It was also causing bidirectional coupling between model fields and
expressions which the method level import of OuterRef was a symptom of.
When the pk was a relation field, qs.filter(pk__in=qs) didn't work.
In addition, fixed Restaurant.objects.filter(place=restaurant_instance),
where place is an OneToOneField and the primary key of Restaurant.
A big thank you to Josh for review and to Tim for review and cosmetic
edits.
Thanks to Beauhurst for commissioning the work on this ticket.
Field.rel is now deprecated. Rel objects have now also remote_field
attribute. This means that self == self.remote_field.remote_field.
In addition, made the Rel objects a bit more like Field objects. Still,
marked ManyToManyFields as null=True.
Previously related fields didn't implement get_lookup, instead
related fields were treated specially. This commit removed some of
the special handling. In particular, related fields return Lookup
instances now, too.
Other notable changes in this commit is removal of support for
annotations in names_to_path().
