title: update nth document in a nested array document in mongodb
body: <p>I need to update a document in an array inside another document in Mongo DB</p>
  
  <pre>
  
      {
              "_id" : ObjectId("51cff693d342704b5047e6d8"),
              "author" : "test",
              "body" : "sdfkj dsfhk asdfjad ",
              "comments" : [
                      {
                              "author" : "test",
                              "body" : "sdfkjdj\r\nasdjgkfdfj",
                              "email" : "test@tes.com"
                      },
                      {
                              "author" : "hola",
                              "body" : "sdfl\r\nhola \r\nwork here"
                      }
              ],
              "date" : ISODate("2013-06-30T09:12:51.629Z"),
              "permalink" : "jaiho",
              "tags" : [
                      "jaiho"
              ],
              "title" : "JAiHo"
      }
  
  
  Q1) Update email of 0th element of comments array
  db.posts.update({"permalink" : "haha"},{$set:{"comments.0.email":1}})
  This doesn't throw any exception but doesn't update anything as well
  Q2) Add a field on nth element of comments array number_likes
  db.posts.update({"permalink" : "haha"},{$inc:{"comments.0.num_likes":1}})
  Doesn't work either.
  
  Am I missing something here?
  </pre>