Django-对称外键或ManyToMany中的级联删除？

Question

I had a model that looked something like this:

class Bar(models.Model):
    foo1 = models.ForeignKey(Foo, related_name='bar_foo1')
    foo2 = models.ForeignKey(Foo, related_name='bar_foo2')

    class Meta:
        unique_together = ('foo1', 'foo2')

The problem was that foo1 and foo2 need to have the same behavior when they are inverted. In other words, it's something like: "If you liked foo1 you may want to check foo2 out", which should apply the other way around ("If you liked foo2 you may want to check foo1 out").

So I used a ManyToMany field instead of ForeignKey to make them symmetrical:

class Bar(models.Model):
    foos = models.ManyToManyField(Foo)

Using a signal to make sure there are never more than 2 foos :

def foos_changed(sender, **kwargs):
    if kwargs['instance'].foos.count() > 2:
        raise ValidationError("You can't assign more than two foos.")    

m2m_changed.connect(foos_changed, sender=Bar.foos)

However, this creates another problem, which is in case a Foo in foos is deleted, the Bar just stays there, but it should be deleted as well (which is solved by cascade deletion in ForeignKeys).

How can I solve this? Can I make ForeignKeys symmetrical? Or implement cascade deletion in ManyToMany? Or is there another way to solve this?

Answer 1

I have done this before, the easiest way is to create a pseudo constructor for the model instance that makes sure that item 1 and item 2 are always in order (I used alphabetical but you might as well use id) which makes your uniqueness work ootb.

In the lookups you just look for your item in foo1 or foo2 since you dont mind in which side of the relation it is, which provides the bidirectionality.

With a few helpers here and there you have a smart object that provides recommendations.

Another more extensible approach is to create a table containing only names for recommendation groups and then m2m objects to recommendation groups so you have a group of foos to recommend, the lookup is easier: foo ->group -> many foos