title: NSPredicate: Fetch one of each kind
body: <p>I want to create an <code>NSFetchRequest</code> for objects like this:</p>
  
  <p>The <code>Object</code> is <code>Car</code> which has an <code>attribute</code> <code>color</code>.
  I have four cars:</p>
  
  <pre><code>car1.color = red
  car2.color = red
  car3.color = blue
  car4.color = green
  </code></pre>
  
  <p>I want to create an <code>NSPredicate</code> that selects only one car for each color(it doesn't matter which car it selects.</p>
  
  <p>How can I achieve this?</p>
  
  <p>In fact I'm looking for something similar like a <code>DISTINCT</code> in SQL</p>