Entity Framework Core，左连接

Question

I have these three tables.

UserCredentials table:

ID  Username    Password
------------------------
1   User1       1
2   User2       2
3   User3       3
4   User4       4
5   User5       5

Connection table; here User1 and User2 are foreign keys to the UserCredentials.ID column:

ID  User1   User2
------------------
1   1       2
3   1       3
5   1       4
7   1       5
9   2       5
11  3       4

UserDetails table; here ID is a foreign key to the UserCredentials.ID column

ID  FullName        Avatar  LastSeen
---------------------------------------------------
1   User1 FullName  2       2021-09-09 00:38:00.000
2   User2 FullName  1       2021-09-09 01:38:00.000
3   User3 FullName  4       2021-09-24 04:38:00.000
4   User4 FullName  5       2021-09-24 18:38:00.000
5   User5 FullName  7       2021-09-24 06:40:00.000

DataContext class:

namespace ChatWeb_MVC.DataModel
{
    public partial class ChatWebAppContext : DbContext
    {
        private readonly string _connectionString;

        public ChatWebAppContext(string connectionString)
        {
            _connectionString = connectionString;
        }
        public ChatWebAppContext(DbContextOptions<ChatWebAppContext> options)
            : base(options)
        {
        }
        public virtual DbSet<Connection> Connections { get; set; }
        public virtual DbSet<UserCredential> UserCredentials { get; set; }
        public virtual DbSet<UserDetail> UserDetails { get; set; }
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            if (!optionsBuilder.IsConfigured)
            {
                optionsBuilder.UseSqlServer(_connectionString);
            }
        }
        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            //Model creation code...
        }
    }

    public partial class Connection
    {
        public Connection()
        {
            Messages = new HashSet<Message>();
        }

        public int Id { get; set; }
        public int User1 { get; set; }
        public int User2 { get; set; }

        public virtual UserCredential User1Navigation { get; set; }
        public virtual ICollection<Message> Messages { get; set; }
    }
    public partial class UserCredential
    {
        public UserCredential()
        {
            Connections = new HashSet<Connection>();
        }

        public int Id { get; set; }
        public string Username { get; set; }
        public string Password { get; set; }

        public virtual Message Message { get; set; }
        public virtual UserDetail UserDetail { get; set; }
        public virtual ICollection<Connection> Connections { get; set; }
    }
    public partial class UserDetail
    {
        public int Id { get; set; }

        public string FullName { get; set; }
        public int Avatar { get; set; }
        public DateTime LastSeen { get; set; }

        public virtual UserCredential IdNavigation { get; set; }
    }

    internal ChatModel GetChatHistory(int userID)
    {
        ChatModel chat = new ChatModel();
        chat.UserName = this.UserCredentials.FirstOrDefault(m => m.Id == userID).Username;
        chat.UserID = userID;
        List<ChatWeb_MVC.Models.UserDetail> connections = new List<ChatWeb_MVC.Models.UserDetail>();
        // Problem
        // I currently have login userID, by using It I want to get his/her friend's details and want to store them in `List<UserDetail> Connections`.
        // For example, if the current login user ID is 3, then I want his friend [User ID: 1 and 4] details stored in `List<UserDetail> Connections`.
        // I've tried this shown below, but I'm not getting the result I want:
        var result = from conn in this.Connections
                     join userDetail in this.UserDetails on conn.User1 equals userDetail.Id into UserDetails
                     from m in UserDetails.DefaultIfEmpty()
                     where conn.User1 == userID || conn.User2 == userID
                     select new ChatWeb_MVC.Models.UserDetail
                     {
                         Id = m.IdNavigation.Id,
                         ConnectionID = conn.Id,
                         Username = m.IdNavigation.Username,
                         FullName = m.FullName,
                         Avatar = GetAvatar(m.Avatar),
                         LastSeen = m.LastSeen,
                     };

        chat.Connections = result.ToListAsync().Result;

        return chat;
    }
}

Here the query I used is not giving the result I want.

Data model class:

namespace ChatWeb_MVC.Models
{
    public class ChatModel
    {
        public List<UserDetail> Connections { get; set; }

        public string UserName { get; set; }

        public int UserID { get; set; }
    }
    public class UserDetail
    {
        public int Id { get; set; }
        public int ConnectionID { get; set; }
        public string Username { get; set; }
        public string FullName { get; set; }
        public string Avatar { get; set; }
        public DateTime LastSeen { get; set; }
    }
}

Answer 1

I'm posting an answer, but I want one query instead of three. If anyone has a better solution please post it.

var friends = this.Connections.Where(m => m.User1 == userID).Select(m => new { conID = m.Id, friendID = m.User2 }).ToListAsync().Result;
friends.AddRange(this.Connections.Where(m => m.User2 == userID).Select(m => new { conID = m.Id, friendID = m.User1 }).ToListAsync().Result);

var result = from friend in friends
              join userDetail in this.UserDetails on friend.friendID equals userDetail.Id into UserDetails
              from m in UserDetails.DefaultIfEmpty()
              select new ChatWeb_MVC.Models.UserDetail
              {
                  Id = m.Id,
                  ConnectionID = friend.conID,
                  FullName = m.FullName,
                  Avatar = GetAvatar(m.Avatar),
                  LastSeen = m.LastSeen,
              };