title: Mongoid Rails update error for documents having embedded documents
body: <p>I have a model called User. User embeds_many posts.</p>
  
  <p>For User records having embedded posts, whenever I try to update any other field, I am getting the error</p>
  
  <pre><code>NoMethodError: undefined method `each' for false:FalseClass
  </code></pre>
  
  <p>I am using update as follows</p>
  
  <pre><code>user = User.find('56da7307421aa90ca4000000')
  user.update(likes: 12)
  </code></pre>
  
  <p>If I remove <code>embeds_many :posts</code> from User model file, the above update query works fine.</p>