title: Keeping track of old part of collection and new part
body: <p>I'm using backbone.js, and I've got a collection with does <code>fetch()</code> sometimes. I don't want to pass the option <code>{add: true}</code> because of the way my subviews are created (the collection is looped over and each item is a new row appended to the current table). The one thing that I tried is just emptying the entire table and creating all new rows, but that is so slow since <code>fetch()</code> is run pretty often (when the scroll gets close to the end of the table). It begins to lag.</p>
  
  <p>I'm hoping there's an alternative where I can keep track of the new collection for appending purposes, yet have the whole collection as an entity. One possibility is creating a second collection and then adding it into the first, but I'm not sure how / if I should do that.</p>