title: Select hash id from an array of hashes
body: <p>Let's say I have an array with multiple hashes</p>
  
  <pre><code>[#&lt;Campaign id: 144&gt; , #&lt;Campaign id: 146&gt;]
  </code></pre>
  
  <p>I've stripped out the other fields for simplicities sake, but the object does have many fields.  What I want to end up with is an array of unique hash IDs, for example: <code>[144, 146]</code>.  Of course there are many ways to do this naively, but I want to know what the best way to do it is.  I'm struggling to find a function that was built for this purpose.</p>