为什么在此脚本中猫鼬唯一不起作用

  var child_process = require('child_process');
  // Load required packages
  child_process.exec("mongo test --eval 'db.users.drop();'", function(err){
  var mongoose = require('mongoose');

  console.log(mongoose.version);
  mongoose.connect('mongodb://localhost:27017/test');
  // Define our user schema

  var json = {};
  json.phone = { type: String, required: true, unique: true};
  var UserSchema = new mongoose.Schema(json);
  var Model = mongoose.model('user', UserSchema);

  var jp = new Model({ phone: "123456"});
  mongoose.connection.on('open', function(){
    console.log(jp);
    jp.save(function(err){
      console.log(err);
      var jp2 = new Model({ phone: "123456"});
      console.log(jp2);
      jp2.save(function(err){
        console.log(err);
        process.exit();
      });
    })
  });
});

我很困惑，结果就像

3.8.20
{ phone: '123456', _id: 54856cceb5b40f7a88fcc2af }
null
{ phone: '123456', _id: 54856cceb5b40f7a88fcc2b0 }
null

谢谢您的帮助。