title: mongodb:find and update operator can combine to one sql command?
body: <p>I have a collection:<code>{"_id":1,"a":3,"b":5，“a+b”:0}</code></p>
  
  <p>I want to do this:</p>
  
  <pre><code>y=db.collection.find({"_id":1},{"a":1,"b":1})
  sum=y["a"]+y["b"]
  db.collection.update({"_id":1},{"$set":{"a+b",sum}})
  </code></pre>
  
  <p>so collection will be: <code>{"_id":1,"a":3,"b":5,"a+b":8}</code></p>
  
  <p>can I combine the two operator to one sql command?</p>