Code First添加外键约束

     //学生表实体
    public partial class UserInfo
    {
        [Key]
        public int Sid { get; set; }
        [StringLength(16,MinimumLength=2)]
        [Required, Column("UserName")]
        public string UserName { get; set; }
        [StringLength(10, MinimumLength = 2)]
        [Required, Column("UserNum")]
        public string UserNum { get; set; }
        
        //自定义字段外键
        [ForeignKey("parent")]
        public int Fid { get; set; }
        //导航属性
        public virtual Parents parent { get; set; }

    }

  //家长表
  public class Parents
    {
        [Key]
        public int Sid { get; set; }
        [StringLength(16, MinimumLength = 2)]
        [Required, Column("Father")]
        public string Father { get; set; }
        [StringLength(16, MinimumLength = 2)]
        [Required, Column("Mather")]
        public string Mather { get; set; }

        [Required, Column("Uid")]
        public int Uid { get; set; }
        //导航属性
        public virtual List<UserInfo> userinfo { get; set; }
    }

   
         //重写方法
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            //绑学生表的Fid字段为外键
            modelBuilder.Entity<UserInfo>().HasRequired(a => a.parent).WithMany(a =>a.userinfo).HasForeignKey(a=>a.Fid);
        }
        //或者
         protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            //绑学生表的Fid字段为外键
            modelBuilder.Entity<Parents>().HasMany(a=>a.userinfo).WithRequired(a=>a.parent).HasForeignKey(a=>a.Fid)
        }