如何执行两个集合的联接并填充导航属性

Question

Let's say I have two classes, Teapot and 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; }
}

and

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; }
}

This is my ViewModel:

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

---

For my ViewModel , I need to perform a join on MaterialId and ColorId and to have some navigation propeties like Teapot.Material.Manufacturer included. So, I've tried following queries:

1. This throws "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(); 

2. This seems to ignore the 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(); 

Now I found some answers here that suggest to enumerate and then to perform another select, but I'd rather catch the data in one go.

Answer 1

You're having difficulties including navigation properties because queries with joins or projections ignore eager loading (see this ).

Unfortunately, you will need to do what you seem to be avoiding: pull the data from the database first, then do an additional select to build your ViewModel (which will have the relationships loaded from the original query). The additional select should be a rather trivial operation, though, since you're not performing an additional load from the database, and the enumerable should only contain a single item.

(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();

Source

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