title: How can I mock mongoose.Types.ObjectId
body: <p>I am writing unit test. I need to mock ObjectId. How can I do this?</p>
  <pre><code>const mockTokenDto: CreateUserTokenDto = {
    token: 
      'a172c98424ad6c6269d398de476940e29feacea5f8ab270ecbc45262ec1d6f04a4abd223925bb0759e915f96c7aaf196',
    userId: '5fa1c587ae2ac23e9c46510f',
    expireAt: '2020-11-04T21:06:26.236Z',
  }
  
  
  export class CreateUserTokenDto {
    @IsString()
    token: string;
    @IsString()
    userId: mongoose.Types.ObjectId;
    @IsDateString()
    expireAt: string;
  }
  </code></pre>