title: Rails how to get table results with a join on itself?
body: <p>I have the following <code>Activity</code> table:</p>
  
  <pre><code>ID      TYPE        SKU
  =======================
  1       create      ABC
  2       create      DEF
  3       destroy     DEF
  </code></pre>
  
  <p><strong>How can I get all <em><code>create</code></em> events which DON'T have a <em><code>destroy</code></em> event?</strong></p>
  
  <p>Doing <code>Activity.where(type: 'create')</code> gives me ids 1 and 2.</p>
  
  <p>But I only want the first row since id 2 also has a destroy event.</p>