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MVC出错案例之一：主外键映射失败

今天在编写DomainModel和DomainMapper，最后放到OnModelCreating中运行的时候，给我抛出了如下错误：

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'.

排查了一会儿找到了原因所在。

请看我的Model：

public class t_expert_content
   {
       public string ID { get; set; }  //PK
       public string TypeID { get; set; }
 
       public string Name { get; set; } //问题标题
       public string Publisher { get; set; }  //发布人
       public int? Count { get; set; }  //查看次数
       public bool IsCheck { get; set; }  //是否审核
       public DateTime PublishDate { get; set; } //发布时间
 
       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);
       }
   }

看上去没啥问题，但是问题就出在外键映射上面。

由于t_expert_content的外键TypeID 是t_expert_type表中的主键ID。由于我在映射的时候，写的是：

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

而在下面进行导航属性指定的时候，ExpertType被指定成了HasOptional类型的：

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

导致二者产生了冲突，抛出了开头的错误。

知道了原因，解决方法就好办了：

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);
       }
   }