#7539: Add ON DELETE and ON UPDATE support to Django
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Reporter: glassfordm | Owner:
Status: new | Milestone:
Component: Database layer (models, ORM) | Version: SVN
Resolution: | Keywords: feature
Stage: Design decision needed | Has_patch: 1
Needs_docs: 1 | Needs_tests: 0
Needs_better_patch: 0 |
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Comment (by emulbreh):
Changes between 7539.on_delete.r11706.diff and 7539.on_delete.r11724.diff:
* Updated to work with m2m-refactor
* Moved `delete_objects()` inside `CollectedObjects`:
`CollectedObjects.delete()`
* Got rid of `DeleteQuery.delete_batch_related()`: use the normal code
path for m2m intermediary models. This is required to enable `on_delete`
for foreign keys on intermediary models. But it results in a dramatical
increase of `SELECT` queries. Therefore ..
* Moved `_collect_sub_objects()` to `CollectedObjects` and changed its
algorithm to collect related objects in batches. This reduces the number
of `SELECT` queries for related objects. The delete call in the following
example requires a total of 103 queries in trunk, while the patch only
requires 4 queries: related `C` instances will be looked up in one bulk
query.
{{{
#!python
class A(models.Model): pass
class B(models.Model):
a = models.ForeignKey(A)
class C(models.Model):
b = models.ForeignKey(B)
a = A.objects.create()
for i in xrange(100): B.objects.create(a=a)
a.delete()
}}}
* Introduced a batch (don't select, just delete) option to get rid of
the one remaining extra query for `auto_created` intermediary models.
* Altered `collect()` to collect inheritance parents without additional
queries.
* Moved `CollectedObjects` into its own module, as it now encapsulates
the whole deletion logic and depends on `db.models.sql`.
TODO:
* There is an ugly `contrib.contenttypes` import in `subqueries.py`.
Deleting generic relations should be handled by a custom `on_delete`
handler instead.
--
Ticket URL: <http://code.djangoproject.com/ticket/7539#comment:30>
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