EF4关系的使用格式（MVC）

实体关系	Has	With	ForeignKey
一对一	.HasRequired	.WithMany	.HasForeignKey
一对多	.HasMany	.WithRequired
多对多	.HasMany	.WithMany	MapLeftKey,MapRightKey,ToTable

 在继承DbContext类中，重写下面的方法，对两个POCO之间关系描述。

 protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<User>().HasMany(u => u.Roles)
                .WithMany(r => r.Users)
                .Map(
                m =>
                {
                    m.MapLeftKey("UserId");
                    m.MapRightKey("RoleId");
                    m.ToTable("UserRoles");
                });

            modelBuilder.Entity<Role>().HasMany(r => r.ControllerActions)
                .WithMany(c => c.Roles)
                .Map(
                m =>
                {
                    m.MapLeftKey("RoleId");
                    m.MapRightKey("ControllerActionId");
                    m.ToTable("RoleControllerAction");
                });

联合主键：

1.modelBuilder.Entity<BlogPost_Category>()
2.    .HasKey(it => it.BlogPostId)
3.    .HasKey(it => it.CategoryId);