title: Move a document to another collection with Mongoose
body: <p>I'm trying to move a document to another collection in MongoDB based on this approach <a href="https://stackoverflow.com/a/15115553/3137109">Saving Mongoose object into two collections</a>.</p>
  
  <p>In the example he shows how to create two new objects. My goal is to move it from one collection to another. </p>
  
  <p>To be a bit more specific: In a document I have a list of tasks which eventually get completed and should then be moved inside this document to another array. However I need to be able to query all the unfinished and this should be possible with two collections.</p>