实体框架CodeFirst多对多返回对象null

Question

我有流利的API和实体框架（关系多对多）的问题。当我试图从数据库中获取数据时，在对象Company和pkdClassification上有空值。价值CompanyId和PkdClassyficationId正常返回。

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; } 
} 

而最后一个公司

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


} 

Answer 1

映射在代码中首先一个多一对多的关系，最好的办法是这样的：

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，代码首先将创建一个第三结合表，ClassificationCompanies其中将包括PK两个表，即。 ClassificationId & CompanyId。

如果需要指定结点表和FK列的名字，那么你可以配置的关系，覆盖在你的上下文OnModelCreating方法的使用流利的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代表联结表的实体（如您试图执行的），并建立与Classification和Company的两个一对多关系，但如果您不需要向联结表添加额外的列，则建议使用第一个变体。如果按照这个变型，实体代表结合表会是这样：

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; } 
} 

而且配置：

 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);} 

Answer 2

你能证明你正在使用得到的值查询？我认为这将与被加载的对象有关。如果您在请求导航属性之前执行了ToList（），那么这些值将为空。你必须在你的查询中添加这种要么强制值包括：

.Include(x => x.Company) 

等为每个对象你想要或要求值，而该对象是否仍然活着。即在执行ToList（）之前;在此处更详细地解释：https://msdn.microsoft.com/en-gb/library/vstudio/bb738633%28v=vs.100%29.aspx

根据您的评论，FirstOrDefault（）将执行查询，因此不能再在此点之后填充导航属性。我会试试这个（提供dbContext.company是一组公司对象）：

添加使用System.Data.Entity;

public Company getCompany(int id) { 
    Company data = dbContext.company.Include(x => x.companyPkdClassification .Select(y => y.company)).Include(x => x.companyPkdClassification .Select(y => y.pkdClassification)).Where(i => i.id == id).FirstOrDefault(); 
    return data; 
}