c# - Entity Framework 代码首先多对多返回对象null

我对流畅的api和实体框架（很多对很多的关系）有疑问。当我尝试从数据库获取数据时，对象Company和pkdClassification的值为空。值CompanyId和PkdClassyficationId正确返回。

语境

 //in contrstructor set
 // this.Configuration.LazyLoadingEnabled = true;

      .Entity<CompanyPkdClassification>()
      (c => new { c.PkdClassyficationId, c.CompanyId });

         modelBuilder.Entity<Company>()
            .HasMany(c => c.companyPkdClassification)
            .WithRequired()
            .HasForeignKey(c => c.CompanyId);

         modelBuilder.Entity<PkdClassification>()
            .HasMany(c => c.companyPkdClassification)
            .WithRequired()
            .HasForeignKey(c => c.PkdClassyficationId);}

pkd分类

public class PkdClassification
{
    [Key]
    [DisplayName("Id")]
    public int id { get; set; }
   ...
    public virtual ICollection<CompanyPkdClassification> companyPkdClassification { get; set; }
}

companyPkd分类

public class CompanyPkdClassification
{
    [ForeignKey("company")]
    public int CompanyId { get; set; }

    [ForeignKey("pkdClassification")]
    public int PkdClassyficationId { get; set; }

    public virtual ICollection<Company> company { get; set; }
    public virtual ICollection<PkdClassification> pkdClassification { get; set; }
}

最后一家公司

public class Company : baseModel
{
   ...
virtual ICollection<CompanyPkdClassification> companyPkdClassification { get; set; }


}

最佳答案

在Code First中映射多对多关系的最佳方法是：

public class Classification
{
  [Key]
  [DisplayName("Id")]
  public int id { get; set; }
  ...
  public virtual ICollection<Company> Companies{ get; set; }
}

public class Company
{
  public int Id { get; set; }
  ...
  public virtual ICollection<Classification> Classifications { get; set; }
}

默认情况下，Code-First将创建第三个联接表ClassificationCompanies，它将由两个表的PK组成。 ClassificationId和CompanyId。

如果需要指定联结表和FK列的名称，则可以使用Fluent Api在上下文中配置覆盖OnModelCreating方法的关系：

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{

   modelBuilder.Entity<Classification>()
               .HasMany<Company>(s => s.Companies)
               .WithMany(c => c.Classifications)
               .Map(cs =>
                        {
                            cs.MapLeftKey("ClassificationId");
                            cs.MapRightKey("CompanyId");
                            cs.ToTable("ClassificationCourses");
                        });

}

另一种方法是创建一个表示联结表的实体（如您尝试做的那样），并使用Classification和Company稳定两个一对多关系，但是如果您不需要向联结添加额外的列表，建议使用第一个变体。如果遵循此变体，则表示联结表的实体将如下所示：

public class CompanyPkdClassification
{
   public int CompanyId { get; set; }

   public int PkdClassyficationId { get; set; }

   public virtual Company Company { get; set; }
   public virtual Classification Classification { get; set; }
}

并配置：

     modelBuilder.Entity<CompanyPkdClassification>().HasKey(c => new { c.PkdClassyficationId, c.CompanyId });

     modelBuilder.Entity<Company>()
        .HasMany(c => c.companyPkdClassification)
        .WithRequired(cc=>Company)
        .HasForeignKey(c => c.CompanyId);

     modelBuilder.Entity<PkdClassification>()
        .HasMany(c => c.companyPkdClassification)
        .WithRequired(cc=>Classification)
        .HasForeignKey(c => c.PkdClassyficationId);}

关于c# - Entity Framework 代码首先多对多返回对象null，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/29589341/