Sort entity by subquery - NSSortdescriptor

I have 3 core data entities:

Entity A and Entity B and User which are related like

Entity A < ------ >> Entity B <<----- > User

I want to sort the entries in Entity A by the number of 开发者_如何学编程entries in entity B for that specific user.

I could do this:

• fetch all Entity A entries
• For each entry - fetch the number of Entity B entries for that entry and for the current user
• Count the number of entries for that Entity A entry store and then sort.

This though seems awfully stupid.

Is there anyway i could sort Entity A with a NSSortDescriptor perhaps by using a Subquery? Or if you know any other way i should solve this?

You can't sort on a collection operator so there is no way to us a sort descriptor to do what you want. Subqueries, like all predicates, simply find objects based on test, they don't sort them.

The easiest solution would be to add a transient attribute, say bCount, to EntityA. Then have the getter method return a count of related EntityB objects:

-(NSNumber *) bCount{
  return [NSNumber numberWithInt:[self.bs count]];
}

Then sort your EntityA objects on the bCount key.