title: ActiveRecord model without primary key
body: <p>I have a ActiveRecord model <code>GPA</code> that doesn't have a primary key:</p>
  
  <pre><code>class GPA &lt; ActiveRecord::Base
  
  end
  </code></pre>
  
  <p>When I try to call <code>GPA.first.to_json</code> I get <code>TypeError: false is not a symbol</code>. I'm guessing that this is due to ActiveRecord trying to lookup the primary key. What is the correct way to implement a model without a primary key?</p>
  
  <p><img src="https://i.stack.imgur.com/rkULk.png" alt="enter image description here"></p>