title: Mongoose - Get list of _ids instead of array of objects with _id
body: <p>I would like to run following query:</p>
  
  <pre><code>Group.find({program: {$in: [...]}}).lean().select('_id')
  </code></pre>
  
  <p>And then <strong>NOT</strong> get following back:</p>
  
  <pre><code>[{_id: ...}, {_id: ...}, {_id: ...}, {_id: ...}]
  </code></pre>
  
  <p><strong>BUT</strong> following:</p>
  
  <pre><code>[..., ..., ..., ...] where ... represents an _id of a Group
  </code></pre>
  
  <p>Of course I could just run the query and then loop through the Groups I get back, but I would like to do it in the query if possible, because that's probably going to be faster.</p>
  
  <p>Thank you guys!</p>