title: Railroad diagram generator fails with "NoMethodError"
body: <p>After making a few modifications to a rails app I am tinkering on, railroad stopped working. The verbose output gives some clues. I wonder if other folks have encountered this and if there are some pointers for fixing this problem. Is it a data modeling error? Is it a problem with railroad? Error log follows...</p>
  
  <pre><code>railroad -vM Loading application environment
  Loading application classes
  Generating models diagram
  
  ...[snip]...
  
      Processing Person
          Processing model association authorships
          Processing model association person_image
          Processing model association publications
  /usr/lib/ruby/gems/1.8/gems/activerecord-2.1.2/lib/active_record/reflection.rb:224:in `derive_class_name': You have a nil object when you didn't expect it! (NoMethodError)
  The error occurred while evaluating nil.class_name
      from /usr/lib/ruby/gems/1.8/gems/activerecord-2.1.2/lib/active_record/reflection.rb:106:in `class_name'
      from /usr/lib/ruby/gems/1.8/gems/railroad-0.5.0/lib/railroad/models_diagram.rb:134:in `process_association'
      from /usr/lib/ruby/gems/1.8/gems/railroad-0.5.0/lib/railroad/models_diagram.rb:102:in `process_class'
      from /usr/lib/ruby/gems/1.8/gems/railroad-0.5.0/lib/railroad/models_diagram.rb:101:in `each'
      from /usr/lib/ruby/gems/1.8/gems/railroad-0.5.0/lib/railroad/models_diagram.rb:101:in `process_class'
      from /usr/lib/ruby/gems/1.8/gems/railroad-0.5.0/lib/railroad/models_diagram.rb:27:in `generate'
      from /usr/lib/ruby/gems/1.8/gems/railroad-0.5.0/lib/railroad/models_diagram.rb:26:in `each'
      from /usr/lib/ruby/gems/1.8/gems/railroad-0.5.0/lib/railroad/models_diagram.rb:26:in `generate'
      from /usr/lib/ruby/gems/1.8/gems/railroad-0.5.0/bin/railroad:47
      from /usr/bin/railroad:19:in `load'
      from /usr/bin/railroad:19
  </code></pre>