Class Question

[h2oloopan]

public class Question {
User ta;
User student;
Enum status;
Id id;
Subject subject;
List messages;
}

[h2oloopan]

public class Message {
User from;
User to;
String uri;
List images;
List sounds;
String text;
}

[ppcheng]

var threadSchema = new Schema({
  createdAt: Date,
  lastUpdatedAt: Date,
  subject: String,
  state: {
    type: String,
    enum: ['open', 'close', 'reopen', 'final', 'cancel']
  },
  student: String,
  ta: [String],
  tags: [String],
  msgList: [
    {
      time: Date,
      from: String,
      to: String,
      text: String,
      zipFileUri: String
    }
  ]
});

@h2oloopan @Shirley-icepoint @jingt06 @MaoningGuo Please let me know ASAP if you think we may need any changes on the above question thread schema.

[ppcheng]

var questionSchema = new Schema({
  createdAt: Date,
  updatedAt: Date,
  subject: String,
  status: {
    type: String,
    enum: ['open', 'close', 'reopen', 'final', 'cancelled']
  },
  askedBy: String,
  answeredBy: [String],
  tags: [String],
  /**
   * hintType Possible Values
   *
   * 1 - Hint ONLY
   * 2 - Full Solution
   */
  hintType: {
    type: Number,
    default: 1
  },
  msgList: [
    {
      time: Date,
      sentBy: String,
      textMsg: String,
      zipFileUri: String
    }
  ]
});

Made some changes based on the DTO class designed by @MaoningGuo;
Please revise this one to see if it meets our expectation;