title: add fields where condition match to nested array
body: <p>I have following <code>users</code> collection</p>
  
  <pre><code>[{
      "_id" : ObjectId("5afadfdf08a7aa6f1a27d986"),
      "firstName" : "bruce",
      "friends" : [ ObjectId("5afd1c42af18d985a06ac306"),ObjectId("5afd257daf18d985a06ac6ac") ]
  },
  {
      "_id" : ObjectId("5afbfe21daf4b13ddde07dbe"),
      "firstName" : "clerk",
      "friends" : [],
  }]
  </code></pre>
  
  <p>and have <code>friends</code> collection</p>
  
  <pre><code>[{
      "_id" : ObjectId("5afd1c42af18d985a06ac306"),
      "recipient" : ObjectId("5afaab572c4ec049aeb0bcba"),
      "requester" : ObjectId("5afadfdf08a7aa6f1a27d986"),
      "status" : 2,
  },
  {
      "_id" : ObjectId("5afd257daf18d985a06ac6ac"),
      "recipient" : ObjectId("5afadfdf08a7aa6f1a27d986"),
      "requester" : ObjectId("5afbfe21daf4b13ddde07dbe"),
      "status" : 1,
  }]
  </code></pre>
  
  <p>suppose I have an user logged in with <code>_id: "5afaab572c4ec049aeb0bcba"</code> and this <code>_id</code> matches the <code>recipient</code> of the <code>friends</code></p>
  
  <p>Now I have to add a field <code>friendsStatus</code> which contains the <code>status</code> from <code>friends</code> collection... And if does not matches the any <code>recipient</code> from the array then its status should be <code>0</code></p>
  
  <p>So when I get all users then my output should be</p>
  
  <pre><code>[{
          "_id" : ObjectId("5afadfdf08a7aa6f1a27d986"),
          "firstName" : "bruce",
          "friends" : [ ObjectId("5afd1c42af18d985a06ac306") ],
          "friendStatus": 2
  },
  {
          "_id" : ObjectId("5afbfe21daf4b13ddde07dbe"),
          "firstName" : "clerk",
          "friends" : [],
          "friendStatus": 0
  }]
  </code></pre>
  
  <p>Thanks in advance!!!</p>