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);
  