MVC出錯案例之一：主外鍵映射失敗

今天在編寫DomainModel和DomainMapper，最后放到OnModelCreating中運行的時候，給我拋出了如下錯誤：

One or more validation errors were detected during model generation:

TinyFrame.Data.DataContext.t_expert_content_ExpertType: : Multiplicity conflicts with the referential constraint in Role 't_expert_content_ExpertType_Target' in relationship 't_expert_content_ExpertType'. Because all of the properties in the Dependent Role are non-nullable, multiplicity of the Principal Role must be '1'.

排查了一會兒找到了原因所在。

請看我的Model：

 public class t_expert_content
    {
        public string ID { get; set; }  //PK
        public string TypeID { get; set; }

        public string Name { get; set; } //問題標題
        public string Publisher { get; set; }  //發布人
        public int? Count { get; set; }  //查看次數
        public bool IsCheck { get; set; }  //是否審核
        public DateTime PublishDate { get; set; } //發布時間

        public string Content { get; set; }
        public int? Order { get; set; }

        public virtual t_expert_type ExpertType { get; set; }
    }

然后是ModelMapper：

 public class t_expert_content_mapper : EntityTypeConfiguration<t_expert_content>
    {
        public t_expert_content_mapper()
        {
            this.ToTable("t_expert_content");

            this.HasKey(x => x.ID);
            this.Property(x => x.ID).HasDatabaseGeneratedOption(DatabaseGeneratedOption.None);
            this.Property(x => x.ID).HasMaxLength(36);

            this.Property(x => x.TypeID).IsRequired().HasMaxLength(36);
            this.Property(x => x.Name).IsRequired().HasMaxLength(500);
            this.Property(x => x.Publisher).IsOptional().HasMaxLength(150);
            this.Property(x => x.Count).IsOptional();
            this.Property(x => x.IsCheck).IsRequired();
            this.Property(x => x.PublishDate).IsRequired();
            this.Property(x => x.Content).IsOptional().HasColumnType("text");
            this.Property(x => x.Order).IsOptional();

            this.HasOptional(x => x.ExpertType)
               .WithMany()
               .HasForeignKey(x => x.TypeID)
               .WillCascadeOnDelete(false);
        }
    }

看上去沒啥問題，但是問題就出在外鍵映射上面。

由於t_expert_content的外鍵TypeID 是t_expert_type表中的主鍵ID。由於我在映射的時候，寫的是：

this.Property(x => x.TypeID).IsRequired().HasMaxLength(36);

而在下面進行導航屬性指定的時候，ExpertType被指定成了HasOptional類型的：

this.HasOptional(x => x.ExpertType)
               .WithMany()
               .HasForeignKey(x => x.TypeID)
               .WillCascadeOnDelete(false);

導致二者產生了沖突，拋出了開頭的錯誤。

知道了原因，解決方法就好辦了：

 public class t_expert_content_mapper : EntityTypeConfiguration<t_expert_content>
    {
        public t_expert_content_mapper()
        {
            this.ToTable("t_expert_content");

            this.HasKey(x => x.ID);
            this.Property(x => x.ID).HasDatabaseGeneratedOption(DatabaseGeneratedOption.None);
            this.Property(x => x.ID).HasMaxLength(36);

            //this.Property(x => x.TypeID).IsRequired().HasMaxLength(36);
            this.Property(x => x.Name).IsRequired().HasMaxLength(500);
            this.Property(x => x.Publisher).IsOptional().HasMaxLength(150);
            this.Property(x => x.Count).IsOptional();
            this.Property(x => x.IsCheck).IsRequired();
            this.Property(x => x.PublishDate).IsRequired();
            this.Property(x => x.Content).IsOptional().HasColumnType("text");
            this.Property(x => x.Order).IsOptional();

            this.HasRequired(x => x.ExpertType)
               .WithMany()
               .HasForeignKey(x => x.TypeID)
               .WillCascadeOnDelete(false);
        }
    }

需要說明的是，上面代碼中的：

this.Property(x => x.TypeID).IsRequired().HasMaxLength(36);

 

可以選擇注釋，也可以選擇不注釋。