TPT mapping in Entity Framework CTP 5

I’ve been working on a little project with MVC 3 and Entity Framework Code-First CTP (community technology preview) 5, and have been implementing TPT (type-per-table) inheritance for the various types of user in the system: Customers, Administrators &c.

Yesterday, I added an Administrator class to my model, and this happened:

The exceptions message reads: The configured property ‘Forename’ is not a declared property on the entity ‘Administrator’. Verify that it has not been explicitly excluded from the model and that it is a valid primitive property.

All very puzzling, as here is my User class:

public class User
{
	public int Id { get; set; }
	[Display(Name = "Forename"), Required]
	public string Forename { get; set; }
	[Display(Name = "Surname"), Required]
	public string Surname { get; set; }
}

and here is my Administrator class:

public class Administrator:User
{
}

(OK, the Administrator class isn’t terribly useful right now, but I’m just fleshing out the schema at the moment.)

Here, finally, is my Repository class, somewhat abbreviated:

public class Repository : DbContext
{
	protected override void OnModelCreating(ModelBuilder modelBuilder)
	{
		modelBuilder.Entity().ToTable("Administrators");
		modelBuilder.Entity().ToTable("Customers");
	}
	public DbSet Administrators { get; set; }
	public DbSet Customers { get; set; }
	public DbSet Users { get; set; }
}

After some futile reading around, I finally decided to roll back to my previous working version, at which point the yellow screen of death disappeared. And it was at this point that the revelation came to me: one of the changes I had made was to alphabetise my DbSet properties, moving Users past Administrators and Customers.

So I went about adding the Administrator class again, but this time keeping the Users property above those for its subclasses:

public class Repository : DbContext
{
	protected override void OnModelCreating(ModelBuilder modelBuilder)
	{
		modelBuilder.Entity().ToTable("Administrators");
		modelBuilder.Entity().ToTable("Customers");
	}
	// Declare superclass first
	public DbSet Users { get; set; }
	public DbSet Administrators { get; set; }
	public DbSet Customers { get; set; }
}

and as if by magic the application started working again.

Moral

When implementing TPT inheritance in CTP 5, declare the superclass DbSet property before its subclasses.