title: MongoDB : how to set a new field equal to the value of another field, for every document in a collection
body: <p>I need to run a migration script to insert a value (already available in each document) into an array of this same document. This has to be done for each documents of my collection (no selection query required)</p>
  
  <p>How to change this:</p>
  
  <pre><code>{
      "_id": ObjectID("5649a7f1184ebc59094bd8b3"),
      "alternativeOrganizer": ObjectID("5649a7f1184ebc59094bd8b1"),
      "myArray": []
  }
  </code></pre>
  
  <p>Into this:</p>
  
  <pre><code>{
      "_id": ObjectID("5649a7f1184ebc59094bd8b3"),
      "alternativeOrganizer": ObjectID("5649a7f1184ebc59094bd8b3"),
      "myArray": [
           ObjectID("5649a7f1184ebc59094bd8b3")
      ]
  }
  </code></pre>
  
  <p>Thanks in advance.</p>