title: Rails has_many association and ActiveRecord#clone
body: <p>shepherd <code>has_many</code> animals. I am trying to clone one of them:</p>
  
  <pre><code>dolly=shepherd.animals.build(sheep.clone)
  </code></pre>
  
  <p>I get error:</p>
  
  <pre><code>undefined method `stringify_keys!' for #&lt;Sheep:0xb6ce154c&gt;
  </code></pre>
  
  <p>why? what is another way to clone dolly so that she would be associated with a shepherd and have sheep's attributes?</p>