How to make recursive ManyToManyField relationships that have extra fields symmetrical in Django?

Question

class Food_Tag(models.Model):
    name = models.CharField(max_length=200)
    related_tags = models.ManyToManyField('self', blank=True, symmetrical=False, through='Tag_Relation')

    def __unicode__(self):
     return self.name

class Tag_Relation(models.Model):
    source = models.ForeignKey(Food_Tag, related_name='source_set')
    target = models.ForeignKey(Food_Tag, related_name='target_set')
    is_a = models.BooleanField(default=False); # True if source is a target
    has_a = models.BooleanField(default=False); # True if source has a target

I want to be able to get the relations between Food_Tags like:

>>> steak = Food_Tag.objects.create(name="steak")
>>> meat = Food_Tag.objects.create(name="meat")
>>> r = Tag_Relation(source=steak, target=meat, is_a=True)
>>> r.save()
>>> steak.related_tags.all()
[<Food_Tag: meat>]
>>> meat.related_tags.all()
[]

but related_tags is empty for meat. I realize this has to do with the 'symmetrical=False' argument, but how can I set up the model such that 'meat.related_tags.all()' returns all related Food_Tags?

Answer 1

As mentioned in the docs:

• When defining a many-to-many relationship from a model to itself, using an intermediary model, you must use symmetrical=False (see the model field reference).

Thus, it is not (yet?) possible to have a symmetrical, recursive many-to-many relationship with extra fields, in Django. It's a "pick two" sorta deal.

Answer 2

I found this approach made by Charles Leifer which seems to be a good approach to overcome this Django limitation.

Answer 3

Since you didn't explicitly say that they need to be asymmetrical, the first thing I'll suggest is setting symmetrical=True. This will cause the relation to work both ways as you described. As eternicode pointed out, you can't do this when you're using a through model for the M2M relationship. If you can afford to go without the through model, you can set symmetrical=True to get exactly the behavior you describe.

If they need to remain asymmetrical however, you can add the keyword argument related_name="sources" to the related_tags field (which you might want to consider renaming to targets to make things more clear) and then access the related tags using meat.sources.all().

Answer 4

To create a symmetrical relationship, you have two options:

1) Create two Tag_Relation objects - one with steak as the source, and another with steak as the target:

>>> steak = Food_Tag.objects.create(name="steak")
>>> meat = Food_Tag.objects.create(name="meat")
>>> r1 = Tag_Relation(source=steak, target=meat, is_a=True)
>>> r1.save()
>>> r2 = Tag_Relation(source=meat, target=steak, has_a=True)
>>> r2.save()
>>> steak.related_tags.all()
[<Food_Tag: meat>]
>>> meat.related_tags.all()
[<Food_Tag: steak]

2) Add another ManyToManyField to the Food_Tag model:

class Food_Tag(models.Model):
    name = models.CharField(max_length=200)
    related_source_tags = models.ManyToManyField('self', blank=True, symmetrical=False, through='Tag_Relation', through_fields=('source', 'target'))
    related_target_tags = models.ManyToManyField('self', blank=True, symmetrical=False, through='Tag_Relation', through_fields=('target', 'source'))

class Tag_Relation(models.Model):
    source = models.ForeignKey(Food_Tag, related_name='source_set')
    target = models.ForeignKey(Food_Tag, related_name='target_set')

As a note, I'd try to use something more descriptive than source and target for your through model fields.

Answer 5

The best solution of this problem (after many investigations) was to manually create symmetrical db record on save() call. This results in DB data redundancy, of course, because you create 2 records instead of one. In your example, after saving Tag_Relation(source=source, target=target, ...) you should save reverse relation Tag_Relation(source=target, target=source, ...) like this:

class Tag_Relation(models.Model):
    source = models.ForeignKey(Food_Tag, related_name='source_set')
    target = models.ForeignKey(Food_Tag, related_name='target_set')
    is_a = models.BooleanField(default=False);
    has_a = models.BooleanField(default=False);

    class Meta:
        unique_together = ('source', 'target')

    def save(self, *args, **kwargs):
        super().save(*args, **kwargs)

        # create/update reverse relation using pure DB-level functions
        # we cannot just save() reverse relation because there will be a recursion
        reverse = Tag_Relation.objects.filter(source=self.target, target=self.source)
        if reverse.exists():
            reverse.update(is_a=self.is_a, has_a=self.has_a)
        else:
            Tag_Relation.objects.bulk_create([
                Tag_Relation(source=self.target, target=self.source, is_a=self.is_a, has_a=self.has_a)
            ])

The only disadvantage of this implementation is duplicating Tag_Relation entry, but except this everything works fine, you can even use Tag_Relation in InlineAdmin.

UPDATE Do not forget to define delete method as well which will remove reverse relation.