title: Understanding _ids array in CakePHP model data
body: <p>Using CakePHP v3.1 w/ Postgres DB. When I retrieve records with associations I often see an extra array of <code>_ids</code>. Something like this:</p>
  
  <pre><code>    ...
      (int) 26 =&gt; [
          'agency_id' =&gt; (int) 23,
          'routes' =&gt; [
              '_ids' =&gt; (int) 2
          ]
      ]
  </code></pre>
  
  <p>Or sometimes:</p>
  
  <pre><code>     '_ids' =&gt; Array (
          0 =&gt; 1
          1 =&gt; 5
          2 =&gt; 3
          3 =&gt; 4
       )
  ]
  </code></pre>
  
  <p>I would like to understand:</p>
  
  <ol>
  <li>How and why do these magic <code>_ids</code> appear?</li>
  <li>Is there a way to control or prevent that behavior?</li>
  </ol>