title: Rails 3 Associations for Traffic Data without Foreign Key
body: <p>I have to define an association that doesn't seem to fit in well to the "has_one / belongs_to" bucket very well.</p>
  
  <p>The situation is this, I have a table whereby each row corresponds to monthly statistics for a given month and year.  I'd love to be able to define certain associations on my model such as <code>record.prior_month</code> or <code>record.prior_year</code> which would correspond to the prior month / year of the current record.</p>
  
  <p>I can't think of any clever way to do this as it doesn't make any sense to maintain foreign keys that would have to be updated every month for tons of records.</p>
  
  <p>I can always handle the logic in the controller, but I'd prefer to keep it with the model if I could.</p>
  
  <p>Thanks!
  Mike</p>