title: mongoose router find by reference properety
body: <p>I am working in my project I already wrote the server.js file for backend
  this is my mongoose schema and model</p>
  <pre><code>const BookSchema = new mongoose.Schema({
  title: {
    type: String,
    required: true
  },
  isbn: {
    type: String,
    required: true
  },
  author: {
    type: String,
    //required: true
  },
  description: {
    type: String
  },
  published_date: {
    type: Date
  },
  publisher: {
    type: String
  },
  updated_date: {
    type: Date,
    default: Date.now
  }
  });
  
  // make data model from schema
  const Book = mongoose.model('book', BookSchema);
  </code></pre>
  <p>this is my router</p>
  <pre><code>router.get('/:id', (req, res) =&gt; {
  
  
   Book.findById(req.params.id)
  .then(book =&gt; res.json(book.title))
  .catch(err =&gt; res.status(404).json({ nobookfound: 'No Book found' }));
  });
  </code></pre>
  <p>How can I pass Book.isbn through get request to get Book.title</p>