title: Mongo: How do I insert another subdocument to an existing document
body: <p>I've started to learn Mongo. Given the following collection, say called posts, how would I go about inserting a new comment to an already existing document? The examples I've seen on the mongo website are for "simple" collections.  Thanks for the help.</p>
  
  <pre><code>{ "_id" : ObjectId( "510a3c5382d395b70b000034" ),
  
    "authorId" : ObjectId( "..." ),
    "comments" : [ 
      { "_id" : ObjectId( "..." ),
        "authorId" : ObjectId( "..." ),
        "content" : "",
        "createdAt" : Date(...) } ],
    "content" : "Some" } 
  </code></pre>