objective c - What is the best way to create derived properties -

I have a data-modal with many-many relationships, using an example of an employee database that says unit partition Regarding the department, which in turn relates to the employee. Employee has a specialty salary, how would be best for a specialty at the division level, which has been derived from the salary specialties. For example, average salary or maximum salary

I need those features so that the list of departments can be sorted out.

Take a look at core data frequently asked questions. If you can not do this, you try to do the keyPathsForValuesAffectingValueForKey: move. If this is not viable, then you have to use KVO to see the changes in the key path, which comes from the value.