title: Core Data: Observing changes in related entities
body: <p>I have a sample managed object model depicted in the image below.  What I would like to happen is this: When the object that is the value for the currency relationship in a Bar object is changed, to automatically have that same object be set as the currency relationship in all Foos that are related to that Bar through the foos relationship.</p>
  
  <p><img src="https://i.stack.imgur.com/flhN4.png" alt="enter image description here"></p>
  
  <p>Am I right to understand that this can be done through KVO?  My inclination was to start by adding this to the Foo.m:</p>
  
  <pre><code>+ (NSSet *)keyPathsForValuesAffectingCurrency {
      return [NSSet setWithObject:@"bar.currency"];
  }
  </code></pre>
  
  <p>but I can't figure out if this is right or how I would proceed further.  Thanks for any advice.</p>