title: MongoDB : How to add an `id` field to a collection?
body: <p>I want to add an <code>id</code> field to a collection,the number starts from <code>1</code>.
  For example,
  there is a collection articles：</p>
  
  <pre><code>{"_id" : ObjectId("98r498jfgragaja9ra4"),"titile":"foo","content":"foo..."}
  {"_id" : ObjectId("498aut89auhgua4940q"),"titile":"bar","content":"bar..."}
  {"_id" : ObjectId("30ua9ah984yhtauiga9"),"titile":"hello","content":"hello..."}
  </code></pre>
  
  <p>after adding <code>id</code> field, the result looks like this: </p>
  
  <pre><code>{"_id" : ObjectId("98r498jfgragaja9ra4"),"titile":"foo","content":"foo...","id":"1"}
  {"_id" : ObjectId("498aut89auhgua4940q"),"titile":"bar","content":"bar...","id":"2"}
  {"_id" : ObjectId("30ua9ah984yhtauiga9"),"titile":"hello","content":"hello...","id":"3"}
  </code></pre>
  
  <p>How to do it?</p>