title: Getting a distinct aggregation of an array field across indexes
body: <p>I'm trying to learn MongoDB and how it'd be useful for analytics for me. I'm simply playing around with the JavaScript console available on their website and have created the following items:</p>
  
  <pre><code>{"title": "Cool", "_id": {"$oid": "503e4dc0cc93742e0d0ccad3"}, "tags": ["twenty", "sixty"]}
  {"title": "Other", "_id": {"$oid": "503e4e5bcc93742e0d0ccad4"}, "tags": ["ten", "thirty"]}
  {"title": "Ouch", "_id": {"$oid": "503e4e72cc93742e0d0ccad5"}, "tags": ["twenty", "seventy"]}
  {"title": "Final", "_id": {"$oid": "503e4e72cc93742e0d0ccad6"}, "tags": ["sixty", "seventy"]}
  </code></pre>
  
  <p>What I'd like to do is query so I get a list of unique tags for all of these objects. The result should look something like this:</p>
  
  <pre><code>["ten", "twenty", "thirty", "sixty", "seventy"]
  </code></pre>
  
  <p>How do I query for this? I'm trying to <code>distinct()</code> it, but the call always fails without even querying.</p>