Entity Framework nested navigation properties count only

Question

I have three model classes

public class Item1{

    public int Id;

    public List<Item2> Item2List { get; set; }
}

public class Item2{
    public int Id;

    //this is the FK
    public int Item1Id {get;set;}

    public Item1 Item1 {get;set;}

    //Not in db. Ignored field in EntityTypeConfiguration
    public int Item3Count;

    public List<Item3> Item3List { get; set; }
}

public class Item3{
    public int Id;

    //this is the FK
    public int Item2Id {get;set;}

    public Item2 Item2 {get;set;}    
}

I want to return the list of Item1 along with list of associated Item2, and load the COUNT of Item3List associated with Item 2 without loading the Item3List.

Here is what I am doing right now:

public IEnumerable<Item1> GetItems()
{
    return base.Query().Include(item1 => item1.Item2List.Select(item2 => item2.Item3List)).ToList();
}

This returns me the list of all 3 objects Item1, Item2 and Item3. But I only need the count of Item3List in Item3Count, and not the entire Item3List list. How can I achieve that? I tried this below, but it throws error.

return base.Query().Include(item1 => item1.Item2List.Select(item2 => new Item2 {
Item3Count = item2.Item3List.Count()
})).ToList();

The Include path expression must refer to a navigation property defined on the type. Use dotted paths for reference navigation properties and the Select operator for collection navigation properties. Parameter name: path

Answer 1

What you want is not possible. Of course you can't populate a not-mapped property in an EF LINQ query, because that's the idea of not mapping it. But you already knew that.

What you'd really like to do is something like this:

context.Item1s.Select(item1 => new Item1
{
    Id = item1.Id,
    Item2s = item1.Item2List.Select(item2 => new Item2List
    {
        Id = item2.Id,
        Item3Count = item2.Item3List.Count()
    })
})

But EF doesn't allow you to construct an entity object in an EF query.

The alternatives are not appealing.

You could build a structure of anonymous types ...

context.Item1s.Select(item1 => new
{
    Item1 = item1,
    Item2s = item1.Item2List.Select(item2 => new
    {
        Item2 = item2,
        Item3Count = item2.Item3List.Count()
    })
})

... and use this to construct a list of Item1 objects, each having their Item2List s of Item2 s with Item3Count values.

Better, but still not close to what would be ideal, is to use AutoMapper and map the entities to DTOs:

Mapper.CreateMap<Item1,Item1Dto>();
Mapper.CreateMap<Item2,Item2Dto>();

You can use AutoMapper's flattening feature to populate Item3Count . To do this, Item2Dto should have a property Item3ListCount and AutoMapper will translate this to Item3List.Count() .

Answer 2

You are not using the ef conventions. Refer to https://msdn.microsoft.com/en-us/data/jj819164.aspx for that. You have to do something like this:

---

public class Item1{

  public int Item1Id{get;set;}

  public ICollection<Item2> Item2s{ get; set; }
}

public class Item2{

  public int Item2Id{get;set;}

  //Not in db. Ignored field in mapping
  [NotMapped]
  public int Item3Count => Item3s.Count;

  //this is the FK
  public int Item1Id{get;set;}

  public Item1 Item1 {get;set;}

  public ICollection<Item3> Item3s{ get; set; }
}

public class Item3{
  public int Item3Id;

  //FK
  public int Item2Id {get;set;}

  public Item2 Item2 {get;set;}

}

---

Now:

public IEnumerable<int> GetCounts()
{
   //get the items1 including its List of item2
   var items1 = base.Query().Include(item1 => item1.Item2s);

   //get the counts
   var counts = items1.Select(item1 => item1.Item2s.Select(item2 => item2.Item3Count));

   //now in counts you have what you want, do what you please with it
   //and return it
   return counts;

}

---

I haven't tried this by it should works for you.