实体框架核心，左连接

Posted 2023-04-12

【中文标题】实体框架核心，左连接

【英文标题】：Entity Framework Core, left join

【发布时间】：2021-11-18 08:07:36

【问题描述】：

我有这三张桌子。

UserCredentials表：

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

Connection 表；这里User1 和User2 是UserCredentials.ID 列的外键：

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

UserDetails 表；这里ID 是UserCredentials.ID 列的外键

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 类：

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;
    

这里我使用的查询没有给出我想要的结果。

数据模型类：

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; 
    

【问题讨论】：

你共享的模型和表不一样...

这行conn.User1 equals userDetail.Id 好像有问题！ User1 和 UserDetail.Id 一样吗？

@Transcendent 它可以是 Connection 表中 User1 User2 中的任何人。

查询中的ChatModel 在哪里？为什么在映射模型类中没有导航属性？

@GertArnold 我已经更新了代码。

【参考方案1】：

我发布了一个答案，但我想要一个查询而不是三个。 如果有人有更好的解决方案，请发布。

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,
              ;