title: Include modules depending on instant type
body: <p>I have an <code>Identity</code> model, which has many <code>types</code> (say, <code>student</code>, <code>teacher</code>). Each of these types has many unique methods defined in different modules, as well as many shared methods. </p>
  
  <p>I would like to include certain modules depending on instant types to avoid naming conflict. Something like:</p>
  
  <pre><code>class Identity &lt; ActiveRecord::Base
   if instant.type =='student'
     include Student
   if instant.type == 'teacher'
     include Teacher
   end
  end
  </code></pre>
  
  <p>How should I do this?</p>