EF 4.3.1 Include inherited navigation properties in LinqToEntities query

Question

I'm trying to setup a simple inheritance scenario with EF 4.3.1 using code first approch and fluent configuration.

I've created an abstract base type 'A' with a one-to-one navigation property and an inherited class 'AA' also with a one-to-one navigation property has following :

public abstract class A
{
    public Guid ID { get; set; }
    public B ChildB { get; set; }
}

public class AA : A
{
    public C ChildC { get; set; }
}

public class B
{
    public Guid ID { get; set; }
    public A Parent { get; set; }
}

public class C
{
    public Guid ID { get; set; }
    public AA Parent { get; set; }
}

public class AConfiguration : EntityTypeConfiguration<A>
{
    public AConfiguration()
    {
        this.HasRequired(o => o.ChildB)
            .WithRequiredPrincipal(o => o.Parent);

        this.Map(o =>
        {
            o.ToTable("A");
        });
    }
}

public class AAConfiguration : EntityTypeConfiguration<AA>
{
    public AAConfiguration()
    {
        this.HasRequired(o => o.ChildC)
            .WithRequiredPrincipal(o => o.Parent);

        this.Map(o =>
        {
            o.ToTable("AA");
        });
    }
}

public class BConfiguration : EntityTypeConfiguration<B>
{
    public BConfiguration()
    {
        this.HasRequired(o => o.Parent)
            .WithRequiredDependent(o => o.ChildB);

        this.Map(o =>
        {
            o.ToTable("B");
        });
    }
}

public class CConfiguration : EntityTypeConfiguration<C>
{
    public CConfiguration()
    {
        this.HasRequired(o => o.Parent)
            .WithRequiredDependent(o => o.ChildC);

        this.Map(o =>
        {
            o.ToTable("C");
        });
    }
}

public class DataContext : DbContext
{
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Configurations.Add<A>(new AConfiguration());
        modelBuilder.Configurations.Add<AA>(new AAConfiguration());
        modelBuilder.Configurations.Add<B>(new BConfiguration());
        modelBuilder.Configurations.Add<C>(new CConfiguration());
    }

    public DbSet<AA> AASet { get; set; }
    public DbSet<B> BSet { get; set; }
    public DbSet<C> CSet { get; set; }
}

When I try to get my data back with the first level of navigation property, it works as expected :

... dataContext.AASet.Include("ChildB") ...

But when I try to include the navigation property of the inherited type like following :

... dataContext.AASet.Include("ChildC") ...

I get an EntityCommandCompilationException at runtime with the following inner exception message :

The ResultType of the specified expression is not compatible with the required type. The expression ResultType is 'Transient.reference[...A]' but the required type is 'Transient.reference[...AA]'. Parameter name: arguments[0]

Has anybody encountered a similar issue ?

I am probably missing something but I can't see what's wrong with this sample.

What can I do to get my model works as expected ?

Malvina

Answer 1

No, you don't miss anything. Actually you ran into an old Entity Framework bug. Your second query can be written like this:

var result = dataContext.ASet.OfType<AA>().Include("ChildC").ToList();

(when you replace your DbSet AASet by ASet).

For this type of eager loading of one-to-one mapped children on inherited types this article applies: http://weblogs.asp.net/johnkatsiotis/archive/2010/04/28/huge-ef4-inheritance-bug.aspx

The bug has been reported long time ago here: https://connect.microsoft.com/VisualStudio/feedback/details/544639/ef4-inheritance-defined-using-queryview-doesnt-work-properly-with-association

The bug still exists in EF 4.3.1. But Microsoft has announced in this thread that the bug is fixed in .NET 4.5 ( = EF 5.0).

The code would work if the relationship is one-to-many instead of one-to-one. Lazy or explicit loading would work as well (also with one-to-one relationship), I believe:

var result = dataContext.ASet.OfType<AA>().ToList();
foreach (var item in result)
    dataContext.Entry(item).Reference(a => a.ChildC).Load();

But this will generate multiple queries. If you don't have performance problems with multiple queries I would prefer the last workaround - until you can migrate to EF 5.0.

Brightman

Thanx for your reply Sauma, but your first workaround generate the same exception and using an explicit load is not an option for my project. – Malvina