#6870: pre_delete should be sent before collecting ForeignKey relationships
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Reporter: wkornewald | Owner: nobody
Status: new | Milestone:
Component: Core framework | Version: SVN
Resolution: | Keywords:
Stage: Design decision needed | Has_patch: 1
Needs_docs: 1 | Needs_tests: 1
Needs_better_patch: 0 |
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Changes (by cgieringer):
* cc: cgieringer (added)
* needs_docs: 0 => 1
* has_patch: 0 => 1
* needs_tests: 0 => 1
Comment:
I agree with the ticket creator that django should send the pre_delete
signal at a time when it is still possible to modify the model in a way
that affects deletion. It seems that the contract of all pre_xx signal
should include informing listeners before any undoable changes have
occurred in the deletion, save, etc.
The most common usage for a correctly behaving pre_delete signal is
apparently to remove related objects to prevent cascade deletion, and the
feature requested in #7539 would eventually provide another means to that
usage. But that possible feature doesn't change the fact that pre_delete
breaks its contract (or what it seems its contract should be) by
collecting the related objects for deletion before informing listeners
that the objects will be deleted, thereby offering no consistent way to
prevent cascade deletion. The only way to get around this limitation is
to define both a custom model delete and manager delete for each model
requiring this functionality, which is a tedious and error-prone
requirement.
Currently the pre_delete signal is sent from
django.db.models.query.delete_objects. The least-complicated place to
send the signal while meeting its contract is from the beginning of
_collect_sub_objects, because this is both the first place that related
objects are seen (as that method traverses relations from the original
model) and the last place that a change to a model's related objects can
be made before they are cached in seen_objects and then sent for deletion.
I have attached a patch which adds a pre_signal argument to
django.db.model.base.Model's _collect_sub_objects. The argument should be
one of Django's signals from django.db.models.signals, and it will be
called at the beginning of the method if the model was not autocreated.
The argument is called pre_signal because although _collect_sub_objects is
presently only used for finding related objects for deletion, it could
possibly be used for some other purpose for which a different pre_xx
signal should be sent. The patch adds _collect_sub_objects(...,
pre_signal=pre_signal) to recursions and _collect_sub_objects(...,
pre_signal=pre_delete) to the two places the method is called. It removes
the sending of pre_delete from delete_objects.
Thank you to all the Django developers, this framework is my favorite
piece of software.
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Ticket URL: <http://code.djangoproject.com/ticket/6870#comment:5>
Django <http://code.djangoproject.com/>
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