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