text: var mongoose = require('mongoose');
  var Schema = mongoose.Schema;
  
  var BookSchema = new Schema({
      isbn: {
          type: String,
          required: true
      },
      title: {
          type: String,
          required: true
      },
      author: {
          type: String,
          required: true
      },
      publisher: {
          type: String,
          required: true
      }
  });
  
  module.exports = mongoose.model('Book', BookSchema);