title: Create a model observer that creates an associated model relationship
body: <p>I have the following models:</p>
  <pre><code>User (id,email,name)
  Group (id,domain)
  GroupUser (id, group_id, user_id)
  </code></pre>
  <p>What I would like to do is anytime a user is created. Find the user's email domain and automatically create the Group and the joining GroupUser record.</p>
  <p>After performing:</p>
  <pre><code>u = User.new
  u.email = 'bob@ge.com'
  u.name = &quot;my name&quot;
  </code></pre>
  <p>What is the right way in the user model/observer to make this happen?</p>