title: mongoDB distinct & where in same query?
body: <p>Let's say I have the following documents</p>
  
  <pre><code>Article { Comment: embedMany }
  
  Comment { Reply: embedMany }
  
  Reply { email: string, ip: string }
  </code></pre>
  
  <p>I want to make a query that selects distinct <code>Reply.ip</code> where <code>Reply.email = xxx</code></p>
  
  <p>Something like this, only it doesn't work..</p>
  
  <pre><code>db.Article.find("Comment.Reply.email" : "xxx").distinct("Comment.Reply.ip")
  </code></pre>
  
  <hr>
  
  <p><strong>JSON export:</strong> </p>
  
  <pre><code>{
     "_id":{
        "$oid":"4e71be36c6eed629c61cea2c"
     },
     "name":"test",
     "Comment":[
        {
           "name":"comment test",
           "Reply":[
              {
                 "ip":"192.168.2.1",
                 "email":"yyy"
              },
              {
                 "ip":"127.0.0.1",
                 "email":"zzz"
              }
           ]
        },
        {
           "name":"comment 2 test",
           "Reply":[
              {
                 "ip":"128.168.1.1",
                 "email":"xxx"
              },
              {
                 "ip":"192.168.1.1",
                 "email":"xxx"
              }
           ]
        }
     ]
  }
  </code></pre>
  
  <p><strong>I run</strong> : <code>db.Article.distinct("Comment.Reply.ip",{"Comment.Reply.email" : "xxx"})</code></p>
  
  <p><strong>I expect</strong> : <code>["128.168.1.1", "192.168.1.1"]</code></p>
  
  <p><strong>I get</strong> : <code>["127.0.0.1", "128.168.1.1", "192.168.1.1", "192.168.2.1"]</code></p>