How do I model a symmetric relationship with django?

Question

Let\'s use the classic example of friends.

class Friendship(models.Model):
    user1 = models.ForeignKey(User, related_name=\'friends1\')
    user2 = models.         


        

Answer 1

Check out the symmetrical option of ManyToManyField in the docs -- sounds like it can do what you want.

For the specific way you're doing it, I'd do something like

class LameUserExtension(User):
    friends = ManyToManyField("self", through=Friendship)

class Friendship(models.Model):
    # the stuff you had here

Answer 2

I found a nice article discussing that some time ago, the basics are the following:

class Person(models.Model):
    name = models.CharField(max_length=100)
    relationships = models.ManyToManyField('self', through='Relationship', 
                                           symmetrical=False, 
                                           related_name='related_to+')

RELATIONSHIP_FOLLOWING = 1
RELATIONSHIP_BLOCKED = 2
RELATIONSHIP_STATUSES = (
    (RELATIONSHIP_FOLLOWING, 'Following'),
    (RELATIONSHIP_BLOCKED, 'Blocked'),
)

class Relationship(models.Model):
    from_person = models.ForeignKey(Person, related_name='from_people')
    to_person = models.ForeignKey(Person, related_name='to_people')
    status = models.IntegerField(choices=RELATIONSHIP_STATUSES)

Note the plus-sign at the end of related_name. This indicates to Django that the reverse relationship should not be exposed. Since the relationships are symmetrical, this is the desired behavior, after all, if I am friends with person A, then person A is friends with me. Django won't create the symmetrical relationships for you, so a bit needs to get added to the add_relationship and remove_relationship methods to explicitly handle the other side of the relationship:

def add_relationship(self, person, status, symm=True):
    relationship, created = Relationship.objects.get_or_create(
        from_person=self,
        to_person=person,
        status=status)
    if symm:
        # avoid recursion by passing `symm=False`
        person.add_relationship(self, status, False)
    return relationship

def remove_relationship(self, person, status, symm=True):
    Relationship.objects.filter(
        from_person=self, 
        to_person=person,
        status=status).delete()
    if symm:
        # avoid recursion by passing `symm=False`
        person.remove_relationship(self, status, False)

Now, whenever we create a relationship going one way, its complement is created (or removed). Since the relationships go in both directions, we can simply use:

def get_relationships(self, status):
    return self.relationships.filter(
        to_people__status=status, 
        to_people__from_person=self)

Source: Self-referencing many-to-many through