Override Delete Function

[cootetom]

Hi, I'm trying to figure out the way django deletes models so that I
can clear the correct references that I need to prior to deleting. So
I have models set up with overrided delete functions so that I can do
clears before the actual delete. However, it appears the delete
functions in a model don't get called in the cascade of deletes so not
each child model gets to do it's clear of linked data before
deleting.

def model1(models.Model):
def delete(self):
self.related_model.clear()
super(model1, self).delete()

def model2(models.Model):
model2 = models.ForeignKey(model2)

def delete(self):
self.another_related_model.clear()
super(model2, self).delete()

So if I do model1.delete() then it will do it's clear but it appears
it won't do the clear from model2? Am I getting this behaviour right
or am I doing something wrong here?

The model class seems like the best place to put delete logic in so
that when it's deleted it clears any data it needs to first.

- Tom

[Ian Lewis]

Tom,

You could try doing this clear logic in a pre_delete signal. You might have to test out the timing of when the signals get called but it should call pre_delete for all deleted models. In this case it would call pre_delete on model2 before model1 but you should be able to get what you are looking for.

http://docs.djangoproject.com/en/dev/ref/signals/#pre-delete

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[cootetom]

Thanks Ian,

I'll give that a go later. I don't suppose it matters what order they
are called in because it's only clearing references to do with the
model instance it's calling from.

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[cootetom]

OK I've tried the pre_delete approach but unfortunately it doesn't
work for what I'm trying here. It appears that django gets a
collection of objects that it is going to delete then it calls
pre_delete on each object before finally deleting the objects. So
django has decided which objects to delete way before the pre_delete
function is called meaning if I do a clear inside that function it
makes no difference to the list of objects that will be deleted.

At the moment the only approach I can think of is to define functions
for deleting models and do the clear logic before calling delete. This
seems really ugly to me but I can't see any other way.

def deleteModal2(obj):
obj.related_model.clear()
obj.delete()

def deleteModal1(obj):
for childObj in obj.related_model.all():
deleteModal2(childObj)

obj.related_model.clear()
obj.delete()

Any more ideas?

[cootetom]

I've settled on a solution for this now which works fine. So I'm
overriding the delete function for my model. In that function I'm
clearing any references that I need to but I'm then also calling
delete manually on any child objects that I also have overridden
delete functions for. This does require an extra step to manually call
delete again on child objects but at least it means I can have delete
logic in a sensible place.

Also found this article on the net which details the options for this
problem nicely
http://stackoverflow.com/questions/2475249/what-are-the-options-for-overriding-djangos-cascading-delete-behaviour