title: populate mongoose without ref as parent of populated object
body: <p>I'm doing a populate on, imagine this mongoose collection:</p>
  
  <pre><code>const userSchema = new Schema({
      name: String,
      lists: [
          _id: {
              type: String,
              ref: List
          }
      ]
  }
  </code></pre>
  
  <p>So, I do a <code>populate</code> on user model, and then as a resulat, I get this:</p>
  
  <pre><code>lists: [{
      _id: {
          _id:"rJ5izylbG"
          ...
      }
  }]
  </code></pre>
  
  <p>I don't want the first <code>_id:</code> and I want it to be just an array of objects. like this:</p>
  
  <pre><code>lists: [
      {
          _id:"rJ5izylbG"
          ...
      }
  ]
  </code></pre>
  
  <p>How can I achieve that with mongoose?</p>