title: why Rails includes ids of nested attributes in forms for update?
body: <p>So imagine you have a form for <code>parent</code> which accepts nested attrs for <code>child</code>, so when you update the <code>parent</code>, rails among <code>child_attributes</code> sends the <code>child's</code> id as well, what for?
  If I set the <code>parent's</code> id in the update action, its not a problem to find the <code>child</code> by the foreign_key, is it? What do I miss?</p>