EntityFramework之多對多關系（四）

上篇介紹了一對多關系，下面介紹下多對多關系代碼編寫。

1、新建model實體，User是用戶類，Role是角色類，由於是多對多關系，必須得有一個中間類，所以產生了UserRole類

    public class User
    {
        public int Id { get; set; }
        public string Name { get; set; }

        public virtual ICollection<UserRole> UserRoles { set; get; }
    }

    public class Role
    {
        public int Id { get; set; }
        public string Name { get; set; }

        public virtual ICollection<UserRole> UserRoles { set; get; }
    }

    public class UserRole
    {
        public int Id { get; set; }
        public int UId { get; set; }
        public int RId { get; set; }

        public virtual User User { set; get; }
        public virtual Role Role { set; get; }
    }

 2、創建一個EntityContext並繼承自DbContext

    public class EntityContext : DbContext
    {
        public EntityContext()
            : base("name=DBConnectionString")
        {

        }

        public DbSet<User> User { get; set; }
        public DbSet<Role> Role { get; set; }
        public DbSet<UserRole> UserRole { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Configurations.AddFromAssembly(Assembly.GetExecutingAssembly());
            base.OnModelCreating(modelBuilder);
        }
    }

 3、在web.config文件中添加數據庫連接字符串

  <connectionStrings>
    <add name="DBConnectionString" connectionString="Data Source=.;Initial Catalog=ManyToMany;Integrated Security=true" providerName="System.Data.SqlClient" />
  </connectionStrings>

 4、接着創建實體映射

    public class UserMap : EntityTypeConfiguration<User>
    {
        public UserMap()
        {
            ToTable("User");
            HasKey(u => u.Id);
        }
    }

    public class RoleMap : EntityTypeConfiguration<Role>
    {
        public RoleMap()
        {
            ToTable("Role");
            HasKey(r => r.Id);
        }
    }

    public class UserRoleMap : EntityTypeConfiguration<UserRole>
    {
        public UserRoleMap()
        {
            ToTable("UserRole");
            HasKey(ur => ur.Id);

            //WillCascadeOnDelete：不使用級聯刪除
            HasRequired(pt => pt.User).WithMany(p => p.UserRoles).HasForeignKey(pt => pt.UId).WillCascadeOnDelete(false);

            HasRequired(pt => pt.Role).WithMany(t => t.UserRoles).HasForeignKey(pt => pt.RId).WillCascadeOnDelete(false);
        }
    }

 5、我們寫一些操作，讓Code First生成數據庫並驗證數據是否新增

    var user = new User() { Name = "Bob" };
    db.Set<User>().Add(user);

    var role = new Role() { Name = "admin" };
    db.Set<Role>().Add(role);

    db.SaveChanges();

    var ur = new UserRole() { RId = role.Id, UId = user.Id };
    db.Set<UserRole>().Add(ur);
    db.SaveChanges();

    //var list = db.Set<User>().Include("UserRoles").ToList();
    //如果需要查出UserRoles下的Role可以這樣
    //避免通過延遲加載需要訪問數據庫(同樣的外鍵實體只會訪問一次,但是比較多的話還是用以下方式)
    var list = db.Set<User>().Include("UserRoles.Role").ToList();

 6、數據庫關系圖