EF6 VS EF Core inheritance

Lost some time with this subtle gotcha, so maybe this will help you spare time.

In EF6 you can configure inheritance with a single type.

An inheritance hierarchy with a single type might seem pointless, but I saw this get used to automatically filter data, which I found pretty clever (to ignore records marked as deleted, the discriminator column was used to only map records with deleted set to false)

Say you want to map SomeEntity only to rows that have value “SomeEntityType” in the field Discriminator:

public class SomeContext : DbContext
{
   protected override void OnModelCreating(DbModelBuilder modelBuilder)
   {
      modelBuilder.Entity<SomeEntity>()
         .Map(m => m.Requires("Discriminator")
            .HasValue("SomeEntityType")
         );
   }
}

Now, transporting the EF6 implementation as is to EF Core syntax won’t work:

// EF Core: incorrect
modelBuilder.Entity<SomeEntity>()
   .HasDiscriminator<string>("Discriminator")
   .HasValue<SomeEntity>("SomeEntityType");

However the gotcha is, that it won’t work, but it won’t crash or warn you either. It will just return all SomeEntities, ignoring the discriminator. Nothing gets added to the WHERE clause in the generated T-SQL.

I spend some time troubleshooting this, until I carefully read the docs again:

EF will only setup inheritance if two or more inherited types are explicitly included in the model

– https://docs.microsoft.com/en-us/ef/core/modeling/inheritance

So in the incorrect mapping, the base type gets setup but nothing else, so no discriminator gets applied (it just ignores the faulty configuration apparently).

You actually need have a separate base and derived type, and map them accordingly, like so:

// EF Core: correct
modelBuilder.Entity<SomeEntityBase>()
   .HasDiscriminator<string>("Discriminator")
   .HasValue<SomeEntityBase>("NotSomeEntityType")
   .HasValue<SomeEntity>("SomeEntityType");

A bit more verbose, but in normal inheritance scenarios you would have had the base type anyway.

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