title: Add a new field to a collection with value of an existing field
body: <p>I'd like to add a new field to a collection, with the value of the new field set to the value of an existing field. </p>
  
  <p>Specifically, I'd like to go from this:</p>
  
  <pre><code># db.foo.findOne()
      {
          "_id"     : ObjectId("4f25c828eb60261eab000000"),
          "created" : ISODate("2012-01-29T16:28:56.232Z"),
          "..."     : ...
      }
  </code></pre>
  
  <p>to this:</p>
  
  <pre><code># db.foo.findOne()
      {
          "_id"      : ObjectId("4f25c828eb60261eab000000"),
          "created"  : ISODate("2012-01-29T16:28:56.232Z"),  
          "event_ts" : ISODate("2012-01-29T16:28:56.232Z"),  #same as created
          "..."      : ...
      }
  </code></pre>
  
  <p>(New documents in this collection won't all have this peculiar redundancy, but I want to do this for my existing documents)</p>