EF Core多对多关系中间表数据插入方法及实体类创建

1. 定义实体类 #

员工实体（Employees） #

public class Employee
{
    public int ID { get; set; }
    public string EmpName { get; set; }

    // 导航属性：关联的项目集合
    public ICollection<EmployeeProject> EmployeeProjects { get; set; } = new List<EmployeeProject>();
}

项目实体（Project） #

public class Project
{
    public int ID { get; set; }
    public string ProjectName { get; set; }

    // 导航属性：关联的员工集合
    public ICollection<EmployeeProject> EmployeeProjects { get; set; } = new List<EmployeeProject>();
}

中间关联实体（EmployeesProject） #

public class EmployeeProject
{
    public int EmpID { get; set; }
    public Employee Employee { get; set; }

    public int ProjectID { get; set; }
    public Project Project { get; set; }
}

2. 在DbContext中配置多对多关系 #

在自定义DbContext的OnModelCreating方法中配置关联规则，指定中间表的复合主键与外键映射：

public class AppDbContext : DbContext
{
    public DbSet<Employee> Employees { get; set; }
    public DbSet<Project> Projects { get; set; }
    public DbSet<EmployeeProject> EmployeeProjects { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // 配置中间表的复合主键
        modelBuilder.Entity<EmployeeProject>()
            .HasKey(ep => new { ep.EmpID, ep.ProjectID });

        // 配置员工与中间表的关联
        modelBuilder.Entity<EmployeeProject>()
            .HasOne(ep => ep.Employee)
            .WithMany(e => e.EmployeeProjects)
            .HasForeignKey(ep => ep.EmpID);

        // 配置项目与中间表的关联
        modelBuilder.Entity<EmployeeProject>()
            .HasOne(ep => ep.Project)
            .WithMany(p => p.EmployeeProjects)
            .HasForeignKey(ep => ep.ProjectID);
    }

    // 配置数据库连接（替换为你的实际连接字符串）
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("你的数据库连接字符串");
    }
}

3. 插入关联数据的几种实现方式 #

方式一：直接创建中间表实体插入 #

已知已存在的员工ID和项目ID时，直接实例化中间表实体添加：

using var context = new AppDbContext();

// 关联ID为1的员工与ID为2的项目
var empProject = new EmployeeProject
{
    EmpID = 1,
    ProjectID = 2
};

context.EmployeeProjects.Add(empProject);
context.SaveChanges();

方式二：通过导航属性添加关联 #

已加载员工或项目实体时，通过导航属性关联：

using var context = new AppDbContext();

// 获取已存在的员工和项目
var employee = context.Employees.Find(1);
var project = context.Projects.Find(2);

// 从员工侧添加关联
employee.EmployeeProjects.Add(new EmployeeProject { Project = project });

context.SaveChanges();

方式三：批量插入关联 #

一次性添加多组关联关系：

using var context = new AppDbContext();

var empProjects = new List<EmployeeProject>
{
    new EmployeeProject { EmpID = 1, ProjectID = 2 },
    new EmployeeProject { EmpID = 1, ProjectID = 3 },
    new EmployeeProject { EmpID = 2, ProjectID = 2 }
};

context.EmployeeProjects.AddRange(empProjects);
context.SaveChanges();

内容的提问来源于stack exchange，提问作者Erum Mirza