title: mongodb ObjectId is treated differetly
body: <p>I tried to query on a document using <code>$in</code> and passing an array of Ids.</p>
  
  <p>On this test I got the values intended.</p>
  
  <pre><code>Document.find({
      documentId: {
          $in: ['598eb5a9957d7427f41d7f08',
              '5a5d863cf9d4d74f2b3d3180'
          ]
      }
  }).then(data =&gt; {
      console.log(data);
  });
  </code></pre>
  
  <p>But when using this, I am just getting an empty result.</p>
  
  <pre><code>Document.aggregate([{
      $match: {
          documentId: {
              $in: ['598eb5a9957d7427f41d7f08',
                  '5a5d863cf9d4d74f2b3d3180'
              ]
          }
      }
  }]).then(data =&gt; {
      console.log(data);
  });
  </code></pre>
  
  <p>I am able to fixed it if I will convert the id explicitly to ObjectId.</p>
  
  <pre><code>Document.aggregate([{
      $match: {
          documentId: {
              $in: [
                  new mongoose.Types.ObjectId(
                      '598eb5a9957d7427f41d7f08'
                  ),
                  new mongoose.Types.ObjectId(
                      '5a5d863cf9d4d74f2b3d3180'
                  )
              ]
          }
      }
  }]).then(data =&gt; {
      console.log(data);
  });
  </code></pre>
  
  <p>Any advice why this is happening?</p>
  
  <p>I want to omit that query on the last code on converting the objectId.</p>
  
  <p>Thanks.</p>