using System.Collections.Generic;

using System;

public class Program

{

    public static void Main()

    {

    }

}

​

public class User

{

    public int UserId {get;set;}

    public string Name {get;set;}

    public List<UserToChatConstraint> Chats {get;set;}

    public List<Message> Messages {get;set;}

}

​

public class Chat

{

    public int ChatId {get;set;}

    public ChatType Type {get;set;}

    public string Title {get;set;}

    public List<UserToChatConstraint> Users {get;set;}

    public List<Message> Messages {get;set;}

}

​

public enum ChatType

{

    Private, Group

}

​

public class UserToChatConstraint

{

    public int UserToChatConstraintId {get;set;}

    public int ChatId {get;set;}

    public Chat Chat {get;set;}

    public int UserId {get;set;}

    public User User {get;set;}

}

​

public class Message

{

    public int MessageId {get;set;}

    public string Text {get;set;}

    public DateTime Date {get;set;}

    public int ChatId {get;set;}

    public Chat Chat {get;set;}

    public int SenderId {get;set;}

    public User Sender {get;set;}

}