[英]Entity Framework CodeFirst many to many return object 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
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; }
}
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
CompanyId
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),并与Classification
和Company
建立两个一对多的关系,但是如果您不需要在联结表中添加额外的列,则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);}
您可以显示用于获取值的查询吗? 我认为这将与延迟加载的对象有关。 如果在请求导航属性之前执行ToList(),则这些值将为null。 您必须通过在查询中添加此值来强制包含该值:
.Include(x => x.Company)
依此类推,为每个您想要或在对象仍然存在时请求其值的对象。 即在您执行ToList()之前; 此处详细说明: https : //msdn.microsoft.com/zh-cn/library/vstudio/bb738633%28v=vs.100%29.aspx
根据您在下面的评论,FirstOrDefault()将执行查询,因此在此之后不再可以填充导航属性。 我会尝试这样做(提供dbContext.company是一组Company对象):
为System.Data.Entity添加using;
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;
}
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