SQLAlchemy self referencial table get the number of grandchilds - sqlalchemy

Question

I got a User model and each user has another user as a parent. Now, to get the number of children (users that belong to a given model instance) I have defined this property:
class User( object ):
#property
def childsCount( self ):
return object_session(self).scalar(
select([func.count(User.users_id)]).where(User.parent_id==self.users_id)
)
...which works ok. What I don't know is how do I get the count of grandchilds? Or even grandgrandchilds.
Any ideas?

Answer 1

Use aliased to composed the deeper level WHERE clauses. In fact you can do it somewhat more generic:
#property
def childrenCount(self):
return self.count_children(0)
#property
def grandchildrenCount(self):
return self.count_children(1)
#property
def grandgrandchildrenCount(self):
return self.count_children(2)
def count_children(self, level=0):
a = [aliased(User) for _ in range(level + 1)]
qry = select([func.count(a[0].users_id)]).where(a[-1].parent_id==self.users_id)
# insert all the intermediate JOINs
for _i in range(level):
qry = qry.where(a[_i].parent_id == a[_i+1].users_id)
return Session.object_session(self).scalar(qry)
Although it looks somewhat cryptic, what it really does is expaned as below (add one alias and where clause for each deeper level):
#property
def children1Count(self):
a0 = aliased(User)
qry = select([func.count(a0.users_id)]).where(a0.parent_id==self.users_id)
return Session.object_session(self).scalar(qry)
#property
def children2Count(self):
a0 = aliased(User)
a1 = aliased(User)
qry = select([func.count(a0.users_id)]).where(a0.parent_id==a1.users_id).where(a1.parent_id==self.users_id)
return Session.object_session(self).scalar(qry)
#property
def children3Count(self):
a0 = aliased(User)
a1 = aliased(User)
a2 = aliased(User)
qry = select([func.count(a0.users_id)]).where(a0.parent_id==a1.users_id).where(a1.parent_id==a2.users_id).where(a2.parent_id==self.users_id)
return Session.object_session(self).scalar(qry)
For first level only you can actually have a nicer query with with_parent:
#property
def childrenCount(self):
return Session.object_session(self).query(User).with_parent(self).count()