c# - Entity Framework 代码首先多对多返回对象 null

Question

我对流畅的 api 和 Entity Framework (多对多关系)有疑问。当我尝试从数据库中获取数据时，对象 Company 和 pkdClassification 具有空值。值 CompanyId 和 PkdClassyficationId 正确返回。

上下文

 //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);}

pkdClassification

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

companyPkdClassification

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; }
}

和最后一家公司

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


}

Answer 1

在 Code First 中映射多对多关系的最佳方式是:

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 将创建第三个连接表，ClassificationCompanies，它将包含两个表的 PK，即。 ClassificationId 和 CompanyId。

如果您需要指定联结表的名称和 FK 列，那么您可以在上下文中配置覆盖 OnModelCreating 方法的关系并使用 Fluent Api:

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(正如您尝试做的那样)并与 Classification 和 Company 建立两个一对多关系，但是如果您不需要向联结表添加额外的列，建议使用第一种变体。如果您遵循此变体，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; }
}

和配置:

     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);}