Design pattern for observing changes in array field within an object

I want to know what is the best design pattern if i want to observe changes in a field of a document which contains an array of object. I know that meteor observeChanges only gives changes in the direct fields.

I have a document like:

Post: {
  title: "dfsfdsf",
  comments: [ {
     text: "213434"
   }, ... 
  ]
}

I want to observeChanges in comment field. What is the best way to do it? Do I have to use observe and compare old and new document ?