Mockgoose:如何在mongoose中模拟错误？

const Mockgoose = require('mockgoose').Mockgoose;
const mongoose = require('mongoose');

const mockgoose = new Mockgoose(mongoose);

before((done) => {
  mockgoose.prepareStorage().then(() => {
    mongoose.connect('mongodb://localhost/testdb', (err) => {
      done(err);
    });
  });
});

it('should handle error', (done) => {
  const User = mongoose.model('User', new mongoose.Schema({
    name: String
  }));

  // 模拟保存错误
  mockgoose.helper.setMockError(User, 'save', new Error('Save error'));

  const user = new User({ name: 'John' });

  user.save((err) => {
    expect(err).to.exist;
    expect(err.message).to.equal('Save error');
    done();
  });
});