Question

We have an entity like Vehicle and three derived entities such as Car, Motorbike and Bicycle.
This inheritance hierarchy is implemented with TPH.

Here are entity mapping conditions:

• __disc__ = car for Car
• __disc__ = motorbike for Motorbike

• __disc__ = bicycle for Bicycle

  How can I derive another child from Vehicle like MotorVehicle with following mapping condition:

• __disc__ = car OR motorbike for MotorVehicle

I’d view in Database like this when I had this structure with TPT:

SELECT    Id
FROM      Vehicles
WHERE     (__Disc__ = N'car') OR (__Disc__ = N'motorbike')

I think this view is not required with TPH.

Please note that I can not change the inheritance like this: Vehicle<– MotorVehicle<– Car. Don’t think about injecting the motor vehicles as the parent of car and other children because Car and Motorbike and Bicycle are already exists. I just want to assign some business to all motor vehicles.

Answer 1

Why couldn’t you introduce a MotorVehicle level in the class hierarchy? You can. It’s just an abstract class, so it doesn’t need a discriminator value. EF hardly notices the class!

I tried both with and without the MotorVehicle class and the database structure and the number of defined discriminators was the same in both cases.

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Edit

This is what I did:

public abstract class Vehicle
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public abstract class MotorVehicle : Vehicle
{
    public int Hp { get; set; }
}

public class Car : MotorVehicle
{ }

public class MotorBike : MotorVehicle
{ }

public class Bicycle : Vehicle
{ }

internal class NestedInheritanceContext : DbContext
{
    public DbSet<Vehicle> Vehicles { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Vehicle>().Property(v => v.Name).IsRequired();
        modelBuilder.Entity<Car>().Map(m => m.Requires("Discriminator")
            .HasValue("car").HasColumnType("char")
            .HasMaxLength(10)
            .IsRequired());
        modelBuilder.Entity<MotorBike>().Map(m => m.Requires("Discriminator")
            .HasValue("motorbike"));
        modelBuilder.Entity<Bicycle>().Map(m => m.Requires("Discriminator")
            .HasValue("bicycle"));
        base.OnModelCreating(modelBuilder);
    }
}