Google app engine has smart feature named back references and I usually iterate them where the traditional SQL's computed column need to be used.

Just imagine that need to accumulate specific force's total hp.

class Force(db.Model):
  hp = db.IntegerProperty()
class UnitGroup(db.Model):
  force = db.ReferenceProperty(reference_class=Force,collection_name="groups")
  hp = db.IntegerProperty()
class Unit(db.Model):
  group = db.ReferenceProperty(reference_class=UnitGroup,collection_name="units")
  hp = db.IntegerProperty()

When I code like following, it was horribly slow (almost 3s) with 20 forces with single group - single unit. (I guess back-referencing force reload sub entities. Am I right?)

def get_hp(self):
    hp = 0
    for group in self.groups:
        group_hp = 0
        for unit in group.units:
            group_hp += unit.hp
        hp += group_hp
    return hp

How can I optimize this code? Please consider that there are more properties should be computed for each force/unit-groups and I don't want to save these collective properties to each entities. :)