EntityFramework之多对多关系（四）

上篇介绍了一对多关系，下面介绍下多对多关系代码编写。

1、新建model实体，User是用户类，Role是角色类，由于是多对多关系，必须得有一个中间类，所以产生了UserRole类

    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、创建一个EntityContext并继承自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文件中添加数据库连接字符串

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

 4、接着创建实体映射

    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：不使用级联删除
            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、我们写一些操作，让Code First生成数据库并验证数据是否新增

    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可以这样
    //避免通过延迟加载需要访问数据库(同样的外键实体只会访问一次,但是比较多的话还是用以下方式)
    var list = db.Set<User>().Include("UserRoles.Role").ToList();

 6、数据库关系图