title: Saving a document in Mongoose, reference id is not stored in the second document
body: <p>When I save a new "experience" document with the model Experience, the experience _id is not saved into the document of the user. So my "experiences" array in the user document remains empty. Why?</p>
  
  <pre><code>const mongoose = require('mongoose');
  
  const ExperienceSchema = mongoose.Schema({
    name: String,
    user: { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
    reviews: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Review' }],
    categories: [{ type: String }],
    });
  
  module.exports = mongoose.model('Experience', ExperienceSchema);
  </code></pre>
  
  <p>==============================================</p>
  
  <pre><code>const mongoose = require('mongoose');
  
  const UserSchema = mongoose.Schema({
    name: String,
    experiences: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Experience' }],
    });
  
  module.exports = mongoose.model('User', UserSchema);
  </code></pre>
  
  <p>=============================================</p>
  
  <pre><code>// Update experience to database
  router.post('/:id', (req, res, next) =&gt; {
    const idexp = req.params.id;
  
    const newExperience = {
      name: req.body.name,
      user: req.user._id,
    };
    Experience.findOneAndUpdate({ _id: idexp }, newExperience, (err, result) =&gt; {
      if (err) {
        return res.render(`/${idexp}/edit`, { errors: newExperience.errors });
      }
      return res.redirect(`/experiences/${idexp}`);
    });
  });
  </code></pre>