title: mongoose: findOne using mongo _id
body: <p>I get that this can be a duplicated question. I looked up at least 10 related questions and answers, but I am still not able to find the document.</p>
  <p>I am trying to get the document using <code>.findOne()</code>. I have the <code>_id</code> that created by MongoDB. But, I get null for every search I try.</p>
  <pre><code>await mongoose.connection.db
                      .collection('testing')
                      .findOne({ _id: req.body.test_id }, (err, result) =&gt; {
                          if (err) {
                              res.status(400);
                          } else {
                              console.log(`whaaaaaahsidufh ${result}`);
                          }
                      });
  </code></pre>
  <p>I tried <code>_id: mongoose.Type.ObjectId(req.body.test_id)</code> and other possible way to search. How can I retrieve the result by using <code>_id</code> on mongoose?</p>