如何执行两个集合的连接并填充导航属性

Question

假设我有两个类、Teapot 和 Cup：

public class Teapot
{
    [Key]
    [Column("Id", TypeName = "int")]
    public int Id         { get; set; }

    public int MaterialId { get; set; }

    public int ColorId    { get; set; }

    [ForeignKey("MaterialId")]
    public virtual Material Material { get; set; }
    [ForeignKey("ColorId")]
    public virtual Color    Color    { get; set; }
}

和

public class Cup
{
    [Key]
    [Column("Id", TypeName = "int")]
    public int Id         { get; set; }

    public int MaterialId { get; set; }

    public int ColorId    { get; set; }

    [ForeignKey("MaterialId")]
    public virtual Material Material { get; set; }
    [ForeignKey("ColorId")]
    public virtual Color    Color    { get; set; }
}

这是我的视图模型：

namespace ViewModel.Data
{
    public class TeapotsWithInfo
    {
        public Model.Data.Teapot Teapot { get; set; }
        public Model.Data.Cup    Cup    { get; set; }
    }
}

对于我的 ViewModel，我需要对 MaterialId 和 ColorId 执行连接，并包含一些导航属性，例如 Teapot.Material.Manufacturer。所以，我尝试了以下查询：

这会抛出 "LINQ to Entities only supports casting EDM primitive or enumeration types"

(from t in db.Teapots
 join c in db.Cups
 on new { t.MaterialId, t.ColorId } equals new { c.MaterialId, c.ColorId }
 where t.Id == id
 select new ViewModel.Data.TeapotsWithInfo { Teapot = t, Cup = c })
    .Include(t => t.Material.Manufacturer).SingleOrDefault();

这好像忽略了Include

 (from t in db.Teapots.Include(t => t.Material.Manufacturer)
 join c in db.Cups
 on new { t.MaterialId, t.ColorId } equals new { c.MaterialId, c.ColorId }
 where t.Id == id
 select new ViewModel.Data.TeapotsWithInfo { Teapot = t, Cup = c }).SingleOrDefault();

现在我在这里找到了一些建议枚举然后执行另一个的答案select，但我宁愿一次性捕获数据。

Answer 1

您在包含导航属性时遇到困难，因为带有连接或投影的查询会忽略预加载（请参阅 this）。

不幸的是，您将需要做您似乎在避免的事情：首先从数据库中提取数据，然后执行额外的 select 来构建您的 ViewModel（它将从原始文件中加载关系询问）。但是，额外的 select 应该是一个相当微不足道的操作，因为您没有从数据库执行额外的负载，并且枚举应该只包含一个项目。

(from t in db.Teapots.Include(t => t.Material.Manufacturer)
 join c in db.Cups
 on new { t.MaterialId, t.ColorId } equals new { c.MaterialId, c.ColorId }
 where t.Id == id
 select new 
  {
     Teapot = t,
     Cup = c,
     Material = t.Material,
     Manufacturer = t.Material.Manufacturer,
  })
.AsEnumerable()
.Select(a => new ViewModel.Data.TeapotsWithInfo 
  { 
     Teapot = a.Teapot, 
     Cup = a.Cup 
  })
.SingleOrDefault();

来源

http://thedatafarm.com/data-access/use-projections-and-a-repository-to-fake-a-filtered-eager-load/

如何执行两个集合的连接并填充导航属性

How to perform a join of two sets and populate navigation properties

c#

asp.net

linq

entity-framework

linq-to-entities