Question

I have a Django project that has two models: Group and Person. Groups can contain either Person objects or other Group objects. Groups cannot form a cycle (i.e. Group A containing Group B containing Group A), which results in a tree structure where Person objects are leaves.

My question is – how can I count all the contained Group objects and Person objects within a high level Group (like the root Group) with as few SQL queries as possible?

A naive approach with O(N) (where N is # of subgroups) SQL queries would be:

def Group(models.Model):
    name = models.CharField(max_length=150)
    parent_group = models.ForeignKey('self', related_name=child_groups, null=True, blank=True)

    # returns tuple (# of subgroups, # of person objects)
    def count_objects(self):
        count = (self.child_groups.count(), self.people.count())
        for child_group in self.child_groups.all():
            # this adds tuples together ( e.g: (1,2) and (1,2) make (2,4) )
            tuple(map(operator.add, count, child_group.count_objects()))

def Person(models.Model):
    user = models.ForeignKey(User)
    picture = models.ImageSpecField(...)
    group = models.ForeignKey('Group', related_name="people")

Is there a way to improve this or should I just store these values within the Group object?

Answer 1

So this is an existing problem that many others have tackled. If you’re using Django, check this out:
http://django-mptt.github.com/django-mptt/index.html