managers.py

class FollowerDescriptor(object):
    """
    A descriptor which provides access to a user's followers.
    """
    def __get__(self, instance, cls):
        if not instance:
            raise AttributeError('Manager must be accessed via instance')

        class RelationshipManager(models.Manager):
            def get_query_set(self):
                return User.objects.filter(following_relationships__to_user=instance)

            def add(self, user):
                raise NotImplementedError("Can't add a follower. Add user to the people the related user is following instead")

            def remove(self, user):
                raise NotImplementedError("Can't remove a follower. Remove user from people the related user is following instead")

        return RelationshipManager()


class FollowingDescriptor(object):
    """
    A descriptor which provides access to users's a user is following.
    """
    def __get__(self, instance, cls):
        if not instance:
            raise AttributeError('Manager must be accessed via instance')
        class RelationshipManager(models.Manager):
            def get_query_set(self):
                return User.objects.filter(follower_relationships__user=instance)

            def add(self, user):
                instance.following_relationships.create(to_user=user)

            def remove(self, user):
                try:
                    relationship = instance.following_relationships.get(to_user=user)
                    relationship.delete()
                except ObjectDoesNotExist:
                    pass

        return RelationshipManager()

models.py

from django.contrib.auth.models import User

class Relationship(models.Model):
    user = models.ForeignKey(User, related_name='following_relationships')
    to_user = models.ForeignKey(User, related_name='follower_relationships')
    reciprocal = models.BooleanField(default=False)
    created_on = models.DateTimeField(auto_now_add=True)

    class Meta:
        unique_together = (('user', 'to_user'))


setattr(User, 'following', managers.FollowingDescriptor())
setattr(User, 'followers', managers.FollowerDescriptor())