title: Make a list from a set of documents in MongoDB collection
body: <p>I have a collection <code>D1</code> like this: </p>
  
  <pre><code>{
     _id: 0,
     author:Jose Fernandez,
     handle: Md8937,
     reference: [
       { item_id: 43, author: Alberto Perez, year: 1910, context: some text },
  
       { item_id: 44, author: Lucas Leys, year: 1990, context: some text },
  
       { item_id: 45, author: Johan Ortiz, year: 2005}
     ]
  }
  {
     _id: 1,
     author: Ramiro Ramirez,
     handle: Gh8765,
     reference: [
       { item_id: 68, author: Mats Valk, year: 1993, context: some text },
  
       { item_id: 74, author: Robert Lucas, year: 1976, context: some text },
  
       { item_id: 80, author: Mark Ljumberg, year: 2005, context: some text}
     ]
  }
  {
     _id: 2,
     author: Feliks Zemdges,
     handle: Yt4573,
     reference: [
       { item_id: 1, author: Gan Zandhi, year: 2015},
  
       { item_id: 2, author: Dayan Wojung, year: 1976, context: some text },
  
     ]
  }
  </code></pre>
  
  <p>I need to make a python list from that contains all <code>handle</code> values for every object in the collection like this:</p>
  
  <pre><code>handles=["Md8937","Gh8765","Yt4573"]
  </code></pre>
  
  <p>How can I do it?</p>