title: query returning count of items in mongodb embedded collection
body: <p>I would like to return the count of an embedded collection in a query in mongodb.</p>
  
  <p>Say I have a collection called profiles</p>
  
  <pre><code>var ProfileSchema = new Schema({
    uname: {
      type: String,
      required: true,
      index: true,
      unique: true
    },
    fname: String,
    lname: String,
    posts: [ObjectId]
  }); 
  </code></pre>
  
  <p>How can I query it so that I select uname, frame, lame, posts.length ?  I don't want to return the posts collection over the wire since users can have hundreds of posts.  Should I even embed the post ids in the profile schema?  Maybe I should just get rid of it since there is already another collection defined for posts, so I'm essentially using Profile.posts as an embedded collection of foreign keys.</p>