title: mongodb how to return list of value only from find query
body: <p>i have a collection placements, each record has fields: placement_id, program_id, category, ...
  i need to find all placements what has program_id = 3 or 5 and only return a list of placement_id. </p>
  
  <p>when i tried this command:</p>
  
  <pre><code>db.placements.find({program_id:{$in: [3, 5]}}, {placement_id:1, _id:0})
  </code></pre>
  
  <p>i got records:</p>
  
  <pre><code>{ "placement_id" : 196 }
  { "placement_id" : 197 }
  { "placement_id" : 198 }
  ...
  </code></pre>
  
  <p>is there any way to return just:</p>
  
  <pre><code>[196, 197, 198...]
  </code></pre>