title: MongoDB : aggregation framework : $match between fields
body: <p>I have a test collection with two documents : </p>
  
  <pre><code>&gt; db.test.find().pretty()
  { "_id" : ObjectId("510114b46c1a3a0f6e5dd7aa"), "a" : 1, "b" : 2 }
  { "_id" : ObjectId("510114c86c1a3a0f6e5dd7ab"), "a" : 3, "b" : 1 }
  </code></pre>
  
  <p>With aggregation framework, I want to get only the documents where a is greater than b.
  $gt get only values in argument not fields...</p>
  
  <pre><code>&gt; db.test.aggregate([{"$match":{"$a":{"$gt":"$b"}}}])
  { "result" : [ ], "ok" : 1 } /* don't work*/
  </code></pre>
  
  <p>Do you have some ideas ?</p>
  
  <p>Thanks in advance</p>
  
  <p>Best regards</p>