title: In MongoDB, how can I retrieve an item and then add to an embedded document within it?
body: <p>I'm trying to do something similar to the following</p>
  
  <p><a href="http://mongoosejs.com/docs/embedded-documents.html" rel="nofollow">http://mongoosejs.com/docs/embedded-documents.html</a></p>
  
  <p>However instead of new BlogPost I'm trying to retrieve then add as follows</p>
  
  <pre><code>function addComment(id, comment, callback) {
      Post.findOne(id, function(err, post) {
      post.comments.push(comment);
      });
  }
  </code></pre>
  
  <p>I'm getting</p>
  
  <pre><code>TypeError: Cannot call method 'call' of undefined
  </code></pre>
  
  <p>What am I doing wrong? I left out the saving code for simplification it is crashing even without trying to save.</p>