title: Fetch within one to many relationship in MagicalRecord
body: <p>I am using MagicalRecord, and I have a one-to-many relationship.
  Say Department have many employees. 
  What's the syntax for finding a specific employee within a specific Department, given a property. (fetch with more than one attribute, but one of them was a one-to-many relationship).</p>
  
  <p>For performance purpose, I would like to avoid retrieving a Set of Employees from a Department, and search that array through a for loop manually...</p>