text: var mongoose = require('mongoose');
  
  var Schema = mongoose.Schema;
  
  var BookSchema = new Schema(
      {
          title: {type: String, required: true},
          author: {type: Schema.Types.ObjectId, ref: 'Author', required: true},
          summary: {type: String, required: true},
          isbn: {type: String, required: true},
          genre: [{type: Schema.Types.ObjectId, ref: 'Genre'}]
      }
  );
  
  // Virtual for book's URL
  BookSchema
      .virtual('url')
      .get(function () {
          return '/catalog/book/' + this._id;
      });
  
  //Export model
  module.exports = mongoose.model('Book', BookSchema);