EF Core中如何设置数据库表自己与自己的多对多关系

本文的代码基于.NET Core 3.0和EF Core 3.0

有时候在数据库设计中，一个表自己会和自己是多对多关系。

在SQL Server数据库中，现在我们有Person表，代表一个人，建表语句如下：

CREATE TABLE [dbo].[Person](
    [PersonID] [int] IDENTITY(1,1) NOT NULL,
    [Name] [nvarchar](50) NULL,
    [Age] [int] NULL,
 CONSTRAINT [PK_Person] PRIMARY KEY CLUSTERED
(
    [PersonID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

其中PersonID列是Person表的主键。

因为一个人会有多个朋友，所以实际上这种人与人之间的朋友关系，是Person表自己和自己的多对多关系，所以我们还要建立一张FriendRelation表，来表示Person表自身的多对多关系，FriendRelation表的建表语句如下：

CREATE TABLE [dbo].[FriendRelation](
    [FriendRelationID] [int] IDENTITY(1,1) NOT NULL,
    [FromPerson] [int] NULL,
    [ToPerson] [int] NULL,
    [Remark] [nvarchar](100) NULL,
 CONSTRAINT [PK_FriendRelation] PRIMARY KEY CLUSTERED
(
    [FriendRelationID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO

ALTER TABLE [dbo].[FriendRelation]  WITH CHECK ADD  CONSTRAINT [FK_FriendRelation_Person_From] FOREIGN KEY([FromPerson])
REFERENCES [dbo].[Person] ([PersonID])
GO

ALTER TABLE [dbo].[FriendRelation] CHECK CONSTRAINT [FK_FriendRelation_Person_From]
GO

ALTER TABLE [dbo].[FriendRelation]  WITH CHECK ADD  CONSTRAINT [FK_FriendRelation_Person_To] FOREIGN KEY([ToPerson])
REFERENCES [dbo].[Person] ([PersonID])
GO

ALTER TABLE [dbo].[FriendRelation] CHECK CONSTRAINT [FK_FriendRelation_Person_To]
GO

其中FriendRelationID列是FriendRelation表的主键，我们可以看到在FriendRelation表中有两个外键关系：

• 外键关系[FK_FriendRelation_Person_From]，通过FriendRelation表的外键列[FromPerson]，关联到Person表的主键列PersonID
• 外键关系[FK_FriendRelation_Person_To]，通过FriendRelation表的外键列[ToPerson]，关联到Person表的主键列PersonID

因此Person表每行数据之间的多对多关系，就通过FriendRelation表的[FromPerson]列和[ToPerson]列建立起来了。

接下来，我们使用EF Core的DB First模式，通过Scaffold-DbContext指令，来生成实体类和DbContext类。

生成Person实体类如下：

using System;
using System.Collections.Generic;

namespace EFCoreSelfMany.Entities
{
    public partial class Person
    {
        public Person()
        {
            FriendRelationFromPersonNavigation = new HashSet<FriendRelation>();
            FriendRelationToPersonNavigation = new HashSet<FriendRelation>();
        }

        public int PersonId { get; set; }
        public string Name { get; set; }
        public int? Age { get; set; }

        public virtual ICollection<FriendRelation> FriendRelationFromPersonNavigation { get; set; }
        public virtual ICollection<FriendRelation> FriendRelationToPersonNavigation { get; set; }
    }
}

可以看到EF Core在实体类Person中生成了两个属性：

• FriendRelationFromPersonNavigation属性，对应了FriendRelation表的外键列[FromPerson]
• FriendRelationToPersonNavigation属性，对应了FriendRelation表的外键列[ToPerson]

所以通过这两个属性我们就能知道一个人有哪些朋友。

生成FriendRelation实体类如下:

using System;
using System.Collections.Generic;

namespace EFCoreSelfMany.Entities
{
    public partial class FriendRelation
    {
        public int FriendRelationId { get; set; }
        public int? FromPerson { get; set; }
        public int? ToPerson { get; set; }
        public string Remark { get; set; }

        public virtual Person FromPersonNavigation { get; set; }
        public virtual Person ToPersonNavigation { get; set; }
    }
}

可以看到EF Core在实体类FriendRelation中也生成了两个属性：

• FromPersonNavigation属性，对应了FriendRelation表的外键列[FromPerson]
• ToPersonNavigation属性，对应了FriendRelation表的外键列[ToPerson]

所以通过这两个属性，我们可以知道一个朋友关系中的两个人（Person表）到底是谁。

最后我们来看看，生成的DbContext类DemoDBContext：

using System;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata;

namespace EFCoreSelfMany.Entities
{
    public partial class DemoDBContext : DbContext
    {
        public DemoDBContext()
        {
        }

        public DemoDBContext(DbContextOptions<DemoDBContext> options)
            : base(options)
        {
        }

        public virtual DbSet<FriendRelation> FriendRelation { get; set; }
        public virtual DbSet<Person> Person { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            if (!optionsBuilder.IsConfigured)
            {
                optionsBuilder.UseSqlServer("Server=localhost;User Id=sa;Password=Dtt!123456;Database=DemoDB");

                optionsBuilder.UseLoggerFactory(new EFLoggerFactory());
            }
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<FriendRelation>(entity =>
            {
                entity.Property(e => e.FriendRelationId).HasColumnName("FriendRelationID");

                entity.Property(e => e.Remark).HasMaxLength();

                entity.HasOne(d => d.FromPersonNavigation)
                    .WithMany(p => p.FriendRelationFromPersonNavigation)
                    .HasForeignKey(d => d.FromPerson)
                    .HasConstraintName("FK_FriendRelation_Person_From");

                entity.HasOne(d => d.ToPersonNavigation)
                    .WithMany(p => p.FriendRelationToPersonNavigation)
                    .HasForeignKey(d => d.ToPerson)
                    .HasConstraintName("FK_FriendRelation_Person_To");
            });

            modelBuilder.Entity<Person>(entity =>
            {
                entity.Property(e => e.PersonId).HasColumnName("PersonID");

                entity.Property(e => e.Name).HasMaxLength();
            });

            OnModelCreatingPartial(modelBuilder);
        }

        partial void OnModelCreatingPartial(ModelBuilder modelBuilder);
    }
}

可以看到在实体类FriendRelation的Fluent API中（黄色高亮部分），设置了Person实体类自己与自己的多对多关系。

然后我们在.NET Core控制台项目中，写了几个方法来做测试：

• ClearTables方法，用于清空Person表和FriendRelation表的数据
• InsertPersonAndFriend方法，用于插入数据到Person表和FriendRelation表
• ShowFriend方法，用于显示Person表数据"张三"的朋友
• DeleteFriend方法，用于删除FriendRelation表数据

代码如下所示：

using EFCoreSelfMany.Entities;
using System;
using Microsoft.EntityFrameworkCore;
using System.Linq;

namespace EFCoreSelfMany
{
    class Program
    {
        //清空Person表和FriendRelation表的数据
        public static void ClearTables()
        {
            using (var dbContext = new DemoDBContext())
            {
                string sql = @"DELETE FROM [dbo].[FriendRelation];
                               DELETE FROM [dbo].[Person];";

                //注意在EF Core 3.0中ExecuteSqlCommand方法已经过时，请用下面的ExecuteSqlRaw方法替代
                dbContext.Database.ExecuteSqlRaw(sql);
            }
        }

        //插入数据到Person表和FriendRelation表
        public static void InsertPersonAndFriend()
        {
            using (var dbContext = new DemoDBContext())
            {
                //插入Person表数据"张三"
                Person personZhangSan = new Person()
                {
                    Name = "张三",
                    Age =
                };

                //插入Person表数据"李四"
                Person personLiSi = new Person()
                {
                    Name = "李四",
                    Age =
                };

                //插入FriendRelation表数据，设置"张三"和"李四"为朋友，注意"张三"是FriendRelation实体类的FromPersonNavigation属性，"李四"是FriendRelation实体类的ToPersonNavigation属性
                FriendRelation friendRelation = new FriendRelation()
                {
                    FromPersonNavigation = personZhangSan,
                    ToPersonNavigation = personLiSi
                };

                dbContext.Person.Add(personZhangSan);
                dbContext.Person.Add(personLiSi);
                dbContext.FriendRelation.Add(friendRelation);

                dbContext.SaveChanges();
            }

            Console.WriteLine("张三 和 李四 已经添加到数据库");
        }

        //显示Person表数据"张三"的朋友
        public static void ShowFriend()
        {
            using (var dbContext = new DemoDBContext())
            {
                //从数据库Person表中找出"张三"，并且使用EF Core的预加载(Eager Loading)，通过Person实体类的FriendRelationFromPersonNavigation属性查询出FriendRelation表的数据，从而找出"张三"的朋友
                //注意，因为"张三"是通过FriendRelation实体类的FromPersonNavigation属性添加到数据库FriendRelation表的，所以这里使用EF Core的预加载(Eager Loading)方法Include时，要使用Person实体类的FriendRelationFromPersonNavigation属性，最后通过FriendRelation实体类的ToPersonNavigation属性从Person表中找出"李四"
                var personZhangSan = dbContext.Person.Where(p => p.Name == "张三").Include(p => p.FriendRelationFromPersonNavigation).ThenInclude(f => f.ToPersonNavigation).First();

                //判断"张三"是否有朋友
                if (personZhangSan.FriendRelationFromPersonNavigation.Count > )
                {
                    Console.WriteLine($"{personZhangSan.Name} 的朋友是 {personZhangSan.FriendRelationFromPersonNavigation.First().ToPersonNavigation.Name}");
                }
                else
                {
                    Console.WriteLine($"{personZhangSan.Name} 没有朋友");
                }
            }
        }

        //删除FriendRelation表数据
        public static void DeleteFriend()
        {
            using (var dbContext = new DemoDBContext())
            {
                //从数据库Person表中找出"张三"，并且使用EF Core的预加载(Eager Loading)，通过Person实体类的FriendRelationFromPersonNavigation属性查询出FriendRelation表的数据
                var personZhangSan = dbContext.Person.Where(p => p.Name == "张三").Include(p => p.FriendRelationFromPersonNavigation).First();
                var friendRelation = personZhangSan.FriendRelationFromPersonNavigation.First();

                //从FriendRelation表中删除数据，也就是删除"张三"和"李四"的朋友关系
                dbContext.FriendRelation.Remove(friendRelation);
                dbContext.SaveChanges();

                Console.WriteLine($"{personZhangSan.Name} 删除了朋友");
            }
        }

        static void Main(string[] args)
        {
            ClearTables();

            InsertPersonAndFriend();

            ShowFriend();

            DeleteFriend();

            ShowFriend();

            Console.WriteLine("按任意键结束...");
            Console.ReadKey();
        }
    }
}

当代码执行完Program类Main方法中的InsertPersonAndFriend方法后，EF Core后台生成的日志如下：

=============================== EF Core log started ===============================
Executed DbCommand (123ms) [Parameters=[@p0='?' (DbType = Int32), @p1='?' (Size = 50)], CommandType='Text', CommandTimeout='30']
SET NOCOUNT ON;
INSERT INTO [Person] ([Age], [Name])
VALUES (@p0, @p1);
SELECT [PersonID]
FROM [Person]
WHERE @@ROWCOUNT = 1 AND [PersonID] = scope_identity();
=============================== EF Core log finished ===============================
=============================== EF Core log started ===============================
Executed DbCommand (18ms) [Parameters=[@p0='?' (DbType = Int32), @p1='?' (Size = 50)], CommandType='Text', CommandTimeout='30']
SET NOCOUNT ON;
INSERT INTO [Person] ([Age], [Name])
VALUES (@p0, @p1);
SELECT [PersonID]
FROM [Person]
WHERE @@ROWCOUNT = 1 AND [PersonID] = scope_identity();
=============================== EF Core log finished ===============================
=============================== EF Core log started ===============================
Executed DbCommand (19ms) [Parameters=[@p2='?' (DbType = Int32), @p3='?' (Size = 100), @p4='?' (DbType = Int32)], CommandType='Text', CommandTimeout='30']
SET NOCOUNT ON;
INSERT INTO [FriendRelation] ([FromPerson], [Remark], [ToPerson])
VALUES (@p2, @p3, @p4);
SELECT [FriendRelationID]
FROM [FriendRelation]
WHERE @@ROWCOUNT = 1 AND [FriendRelationID] = scope_identity();
=============================== EF Core log finished ===============================

可以看到InsertPersonAndFriend方法中，EF Core一共执行了三段SQL语句，前面两段SQL就是在Person表中插入了"张三"和"李四"两行数据，最后一段SQL就是在FriendRelation表中插入了"张三"和"李四"的朋友关系数据。

执行完Program类Main方法中的InsertPersonAndFriend方法后，数据库Person表记录如下：

数据库FriendRelation表记录如下：

控制台输出结果如下：

当代码执行完Program类Main方法中的第一个ShowFriend方法后，EF Core后台生成的日志如下：

=============================== EF Core log started ===============================
Executed DbCommand (13ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
SELECT [t].[PersonID], [t].[Age], [t].[Name], [t0].[FriendRelationID], [t0].[FromPerson], [t0].[Remark], [t0].[ToPerson], [t0].[PersonID], [t0].[Age], [t0].[Name]
FROM (
    SELECT TOP(1) [p].[PersonID], [p].[Age], [p].[Name]
    FROM [Person] AS [p]
    WHERE ([p].[Name] = N'张三') AND [p].[Name] IS NOT NULL
) AS [t]
LEFT JOIN (
    SELECT [f].[FriendRelationID], [f].[FromPerson], [f].[Remark], [f].[ToPerson], [p0].[PersonID], [p0].[Age], [p0].[Name]
    FROM [FriendRelation] AS [f]
    LEFT JOIN [Person] AS [p0] ON [f].[ToPerson] = [p0].[PersonID]
) AS [t0] ON [t].[PersonID] = [t0].[FromPerson]
ORDER BY [t].[PersonID], [t0].[FriendRelationID]
=============================== EF Core log finished ===============================

可以看到EF Core生成了SQL语句，将"张三"和其朋友的数据都从Person表和FriendRelation表查询出来了。

控制台输出结果如下：

当代码执行完Program类Main方法中的DeleteFriend方法后，EF Core后台生成的日志如下：

=============================== EF Core log started ===============================
Executed DbCommand (28ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
SELECT [t].[PersonID], [t].[Age], [t].[Name], [f].[FriendRelationID], [f].[FromPerson], [f].[Remark], [f].[ToPerson]
FROM (
    SELECT TOP(1) [p].[PersonID], [p].[Age], [p].[Name]
    FROM [Person] AS [p]
    WHERE ([p].[Name] = N'张三') AND [p].[Name] IS NOT NULL
) AS [t]
LEFT JOIN [FriendRelation] AS [f] ON [t].[PersonID] = [f].[FromPerson]
ORDER BY [t].[PersonID], [f].[FriendRelationID]
=============================== EF Core log finished ===============================
=============================== EF Core log started ===============================
Executed DbCommand (15ms) [Parameters=[@p0='?' (DbType = Int32)], CommandType='Text', CommandTimeout='30']
SET NOCOUNT ON;
DELETE FROM [FriendRelation]
WHERE [FriendRelationID] = @p0;
SELECT @@ROWCOUNT;
=============================== EF Core log finished ===============================

可以看到EF Core生成了两段SQL语句，第一段SQL是通过"张三"找出FriendRelation表的数据，第二段SQL是将找出的FriendRelation表数据进行了删除。

执行完Program类Main方法中的DeleteFriend方法后，数据库FriendRelation表记录如下：

控制台输出结果如下：

当代码执行完Program类Main方法中的第二个ShowFriend方法后，控制台输出结果如下：

所以我们可以看到，EF Core是支持数据库表自己与自己多对多关系的实体类映射的，当实体类生成好后，其使用方法和普通的多对多关系差不多，没有太大的区别。

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