This required moving the entirety of DELETE SQL generation to the
compiler where it should have been in the first place and implementing
a specialized compiler on MySQL/MariaDB.
The MySQL compiler relies on the "DELETE table FROM table JOIN" syntax
for queries spanning over multiple tables.
Reduced the number of queries required when performing cascade deletion
for a model referenced multiple time by another one by performing an
union of reference lookups.
The non-referenced fields can only be deferred if no deletion signals
receivers are connected for their respective model as connected as these
receivers might expect all fields of the deleted model to be present.
Thanks Ed Morley for the report.
Thanks to Trac alias ppetrid for the report and Tim for the review.
Conflicts:
django/db/models/deletion.py
tests/delete/tests.py
Forward port of 7c3ef19978b36b61db88a519f799f1ce8d019679 from stable/1.8.x
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.
SQLite doesn't work with more than 1000 parameters in a single query.
The deletion code could generate queries that try to get related
objects for more than 1000 objects thus breaking the limit. Django now
splits the related object fetching into batches with at most 1000
parameters.
The tests and patch include some work done by Trac alias NiGhTTraX in
ticket #21205.
