> Sure. I'm trying to set a boolean property on the object based on the
> existence (or absence) of a ManyToMany relationship. So I originally
> thought something like:
>
> def save(self):
> self.has_m2m_thing = bool(self.m2m_relation.count())
> super(MyModel, self).save()
>
> This works fine if you save it twice, but that's not the behavior I
> was looking for :).
Right.
How about something along these lines (a lazy initialization pattern):
1. Change MyModel's has_m2m_thing to m2m_count and make it an integer
field and default it to -1.
2. Add a method and property to MyModel:
def _has_m2m_thing(self):
if self.m2m_count == -1:
# compute count
self.m2m_count = self.m2m_relation.count()
super(MyModel, self).save()
return self.m2m_count > 0
has_m2m_thing = property(_has_m2m_thing)
3. Add this to MyModel.save()
def save(self):
self.m2m_count = -1 # reset count
super(MyModel, self).save()
Caveat: This doesn't work if you add or remove your m2m objects
outside of the Admin (because MyModel.save() won't get called so the
m2m_count won't get reset.) You will have to explicitly reset
m2m_count in such cases by calling MyModel.save().
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