title: Get distinct first words in a string
body: <p>I try to get distinct first words in a field in a database:</p>
  
  <pre><code>db.mycollection.distinct(substr(0, city.indexOf(' ')),{"state":"FirstState"})
  </code></pre>
  
  <p>But that does not work obviosly, but maybe it helps to get an idea about what I want to do.</p>
  
  <p>Say I have two documents:</p>
  
  <pre><code>{
    "_id": "10280",
    "city": "FirstCity is great",
    "state": "FirstState",
    "pop": 2224
  },
  {
    "_id": "10282",
    "city": "SecondCity even greater",
    "state": "FirstState",
    "pop": 5574
  }
  </code></pre>
  
  <p>I then want to get:</p>
  
  <pre><code>["FirstCity","SecondCity"]
  </code></pre>
  
  <p>I.e. get distinct values of <code>city</code> field's <code>first word</code> in array.
  Is there any way to do that?</p>
  
  <p>Best Regards</p>