title: Ruby on rails, `rails generate` with primary_key not the default integer id
body: <p>I have a model object called 'Problem' and I want to be able to call its the 'show' action using the URL  /problems/PROBLEM_NAME.</p>
  
  <p>I think the way to go about this is to change it so the primary_key is no longer the default integer id, instead I want it to be a string of letters.</p>
  
  <p>I have tried to <code>rails g model problem problem_name:primary_key problem_text:text</code>
  but I get the obvious error about having multiple primary keys. </p>
  
  <p>Any ideas?</p>