title: Mongodb query with fields in the same documents
body: <p>I have the following json:</p>
  
  <pre><code>{
    "a1": {"a": "b"},
    "a2": {"a": "c"}
  }
  </code></pre>
  
  <p>How can I request all documents where <code>a1</code> and <code>a2</code> are not equal in the same document?</p>