title: How do I update a Mongo document after inserting it?
body: <p>Let's say I insert the document.</p>
  
  <pre><code>post = { some dictionary }
  mongo_id = mycollection.insert(post)
  </code></pre>
  
  <p>Now, let's say I want to add a field and update it. How do I do that? This doesn't seem to work.....</p>
  
  <pre><code>post = mycollection.find_one({"_id":mongo_id}) 
  post['newfield'] = "abc"
  mycollection.save(post)
  </code></pre>