title: Adding a has_many association to an existing model
body: <p>I have the models Student, Classroom and Course.
  I have a has_and_belongs_to_many association between Student and Classroom, and another has_and_belongs_to_many association between Classroom and Course.</p>
  
  <p>I have already Created the models and the databases are populated.</p>
  
  <p>Now, how do I add a has_many association between Student and Course and one between Course and Student?</p>
  
  <p>Thanks!</p>