Sort Sub Documents in MongoDB

Question

Is there a way to sort sub documents in a mongo query?

For example:

{
  "_id" : ObjectId("4c69d19532f73ad544000001"),
  "content" : "blah blah blah",
  "comments" : [
    {"author": "jim", "content":"comment content 1", "date" : "07-24-1995"},
    {"author": "joe", "content":"comment content 2", "date" : "07-24-1996"},
    {"author": "amy", "content":"comment content 3", "date" : "09-10-1999"}
  ]
}
{
  "_id" : ObjectId("4c69d19532f73ad544000002"),
  "content" : "blah blah blah",
  "comments" : [
    {"author": "jim", "content":"comment content 1", "date" : "07-24-1995"},
    {"author": "joe", "content":"comment content 2", "date" : "07-24-1996"},
    {"author": "amy", "content":"comment content 3", "date" : "07-24-1997"}
  ]
}

I want these documents to be ordered in however way I decide and then have comments ordered within my a document by reverse order of date or any other sorting I want.

Is that possible with mongoDB?

Answer 1

In mongo version 2.4 or above, you can use $sort while updating. The new modifier let you sort sub document.

More information could read this page, http://docs.mongodb.org/manual/reference/operator/sort/

Answer 2

It's not possible directly out of MongoDB, however when you pull the document you can then sort the array as if it was an array of objects, using whatever native sort method your language has. This is how I do comments on my blog with MongoDB.

^{PHP Code}

/* Takes an array of comments and turns them into native objects */
public static function loadMany($pcomments)
{
    $comments = array();
    if(isset($pcomments) && count($pcomments) > 0)
    {
        foreach($pcomments as $key => $comment)
        {
            $comment['data']['index'] = $key;
            $comments[] = Comment::loadOne($comment['data']);
        }
    }
    usort($comments, "comment_compare");
    return $comments;
}

/* Compares comment timestamps */
function comment_compare($a, $b)
{
    if($a->timestamp->sec == $b->timestamp->sec)
    {
        return 0;
    }
    return ($a->timestamp->sec < $b->timestamp->sec) ? -1 : 1;
}