title: Merge values of two fields into one across a collection
body: <p>Is there any way to merge values of two fields into one field or array?
  I have a MongoDB collection like this:</p>
  
  <pre><code>{"_id": 1, "side1": "a", "side2": "b"},
  {"_id": 2, "side1": "c", "side2": "b"},
  {"_id": 3, "side1": "b", "side2": "d"},
  {"_id": 4, "side1": "a", "side2": "d"}
  </code></pre>
  
  <p>How to get an array containing all values of side1 and side2.</p>
  
  <pre><code>{"_id": null, "sides": ["a", "b", "c", "d"]}
  </code></pre>
  
  <p>The id isn't important. I have tried many aggregations and cascading query commands and just can't come up with desired response.</p>