title: Fetch object by property in Core Data
body: <p>In my iPhone project, I want to write a function that checks wether there's an object in my Core Data ManagedObjectContext with a given value for a certain property, say <code>some_property</code>.</p>
  
  <p>If there's already an object with <code>some_property == 12</code>, I want the function to return the object, otherwise, I want to create the object, or at least return <code>nil</code>.</p>
  
  <p>How would I do that?</p>