entity-framework – 首先在Entity Framework代码中检索关联表中的值

我首先使用EF 4.1代码,我正在努力与关联实体并获取在关联表中设置的值.我试着关注帖子： Create code first, many to many, with additional fields in association table.

我的表格如下(全部为复数形式)：

表：产品

Id int
Name varchar(50)

表：规格

Id int
Name varchar(50)

表：产品规格

ProductId int
SpecificationId int
SpecificationValue varchar(50)

我的相关课程：

public class Product : IEntity
{
     public int Id { get; set; }
     public string Name { get; set; }
     public virtual ICollection<ProductSpecification> ProductSpecifications { get; set; }
}

public class Specification : IEntity
{
     public int Id { get; set; }
     public string Name { get; set; }
     public virtual ICollection<ProductSpecification> ProductSpecifications { get; set; }
}

public class ProductSpecification
{
     public int ProductId { get; set; }
     public virtual Product Product { get; set; }
     public int SpecificationId { get; set; }
     public virtual Specification Specification { get; set; }
     public string SpecificationValue { get; set; }
}

我的上下文类：

public class MyContext : DbContext
{
     public DbSet<Product> Products { get; set; }
     public DbSet<Specification> Specifications { get; set; }
     public DbSet<ProductSpecification> ProductSpecifications { get; set; }

     protected override void OnModelCreating(DbModelBuilder dbModelBuilder)
     {
     }
}

我的存储库方法我在哪里打电话(不确定它是否正确)：

public class ProductRepository : IProductRepository
{
     MyContext db = new MyContext();

     public Product GetById(int id)
     {
          var product = db.Products
               .Where(x => x.Id == id)
               .Select(p => new
               {
                    Product = p,
                    Specifications = p.ProductSpecifications.Select(s => s.Specification)
               })
               .SingleOrDefault();

          return null;  // It returns null because I don't know how to return a Product object?
     }
}

这是我要回复的错误：

One or more validation errors were detected during model generation:

System.Data.Edm.EdmEntityType: : EntityType 'ProductSpecification' has no key defined. Define the key for this EntityType.
 System.Data.Edm.EdmEntitySet: EntityType: EntitySet �ProductSpecifications� is based on type �ProductSpecification� that has no keys defined.

什么意思没有定义键？ ProductId和SpecificationId不会分别映射到产品的Id和规范的ID吗？

我如何退回具有所有规格的单一产品？

实体框架将识别ProductId是Product导航属性的外键属性,而SpecificationId是Specification导航属性的外键属性.但该异常会抱怨您的ProductSpecification实体上缺少主键(“Key”=“Primary Key”).每个实体都需要定义一个关键属性.这可以通过约定 – 通过键属性的特定命名 – 或明确使用数据注释或Fluent API来实现.

您的ProductSpecification类没有EF会按惯例识别为键的属性：No Id属性而没有ProductSpecificationId(类名“Id”).

所以你必须明确地定义它.使用数据注释定义它会显示在您链接的帖子中：

public class ProductSpecification
{
    [Key, Column(Order = 0)]
    public int ProductId { get; set; }
    public virtual Product Product { get; set; }

    [Key, Column(Order = 1)]
    public int SpecificationId { get; set; }
    public virtual Specification Specification { get; set; }

    public string SpecificationValue { get; set; }
}

在Fluent API中,它将是：

modelBuilder.Entity<ProductSpecification>()
    .HasKey(ps => new { ps.ProductId, ps.SpecificationId });

两种方式都定义了一个复合键,每个部分同时是Product或Specification表的外键. (您不需要明确定义FK属性,因为EF因其常规友好名称而识别它.)

您可以退回包含所有规格的产品,例如：

var product = db.Products
    .Include(p => p.ProductSpecifications.Select(ps => ps.Specification))
    .SingleOrDefault(x => x.Id == id);