title: Concatenate string values in array in a single field in MongoDB
body: <p>Suppose that I have a series of documents with the following format:</p>
  
  <pre><code>{
      "_id": "3_0",
      "values": ["1", "2"]
  }
  </code></pre>
  
  <p>and I would like to obtain a projection of the array's values concatenated in a single field:</p>
  
  <pre><code>{
      "_id": "3_0",
      "values": "1_2"
  }
  </code></pre>
  
  <p>Is this possible? I have tried <code>$concat</code> but I guess I can't use <code>$values</code> as the array for <code>$concat</code>.</p>