Derived types generating their own ForeignKey field in code first using TPH

Question

I'm using EF Core and implementing inheritance in a table-per-hierarchy (TPH) approach. I've found that once I added the hierarchy objects to my data model the migration is adding a foreign key for the base table and what appears to be a duplicate that the descendant classes are using.

Relationship is from Transaction (parent) to TransactionItem (child).

Object hierarchy:

• TransactionItem (Base class)
  • Attachment
  • Check
  • Envelope
  • Stub

Using Fluent API to define the relationship between Transaction and TransactionItem using the TransactionItem.TransactionId field as the foreign key:

modelBuilder.Entity<TransactionItem>(t =>
{
    t.ToTable("TransactionItem");
    t.HasKey(a => a.Id);
    t.Property(a => a.ConcurrencyStamp)
        .IsRequired()
        .IsConcurrencyToken();
    t.Property(a => a.Created)
        .IsRequired();
    t.HasOne(a => a.Transaction)
        .WithMany(a => a.TransactionItems)
        .HasForeignKey(a => a.TransactionId)
        .OnDelete(DeleteBehavior.Restrict);
    t.HasDiscriminator<string>("ItemType")
        .HasValue<Check>("Check")
        .HasValue<Stub>("Stub")
        .HasValue<Envelope>("Envelope")
        .HasValue<Attachment>("Attachment");
});

The migration Up method is creating the base class table (TPH) with TransactionId and TransactionId2 columns, each relating to the same parent table:

migrationBuilder.CreateTable(
name: "TransactionItem",
columns: table => new
{
    Id = table.Column<int>(nullable: false)
        .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn),
    ConcurrencyStamp = table.Column<string>(nullable: false),
    Created = table.Column<DateTimeOffset>(nullable: false),
    ItemType = table.Column<string>(nullable: false),
    PageName = table.Column<string>(nullable: true),
    Sequence = table.Column<int>(nullable: false),
    TransactionId = table.Column<int>(nullable: false),
    TransactionId2 = table.Column<int>(nullable: true),
    CheckType = table.Column<string>(nullable: true),
    Micr = table.Column<string>(nullable: true),
    MicrValid = table.Column<bool>(nullable: true),
    OcrId = table.Column<string>(nullable: true)
},
constraints: table =>
{
    table.PrimaryKey("PK_TransactionItem", x => x.Id);
    table.ForeignKey(
        name: "FK_TransactionItem_Transaction_TransactionId",
        column: x => x.TransactionId,
        principalTable: "Transaction",
        principalColumn: "Id",
        onDelete: ReferentialAction.Restrict);
    table.ForeignKey(
        name: "FK_TransactionItem_Transaction_TransactionId2",
        column: x => x.TransactionId2,
        principalTable: "Transaction",
        principalColumn: "Id",
        onDelete: ReferentialAction.Restrict);
});

When I look at the ModelSnapshot.cs code I see the following relationships specific to the objects in the inheritance model establishing a foreign key for the derived class entities using the TransactionId2 column.

modelBuilder.Entity("SampleProject.Data.DataModels.TransactionItem", b =>
    {
        b.HasOne("SampleProject.Data.DataModels.Transaction", "Transaction")
            .WithMany("TransactionItems")
            .HasForeignKey("TransactionId");
    });

modelBuilder.Entity("SampleProject.Data.DataModels.Attachment", b =>
    {
        b.HasOne("SampleProject.Data.DataModels.Transaction")
            .WithMany("Attachments")
            .HasForeignKey("TransactionId2");
    });

modelBuilder.Entity("SampleProject.Data.DataModels.Check", b =>
    {
        b.HasOne("SampleProject.Data.DataModels.Transaction")
            .WithMany("Checks")
            .HasForeignKey("TransactionId2");
    });

modelBuilder.Entity("SampleProject.Data.DataModels.Envelope", b =>
    {
        b.HasOne("SampleProject.Data.DataModels.Transaction")
            .WithMany("Envelopes")
            .HasForeignKey("TransactionId2");
    });

modelBuilder.Entity("SampleProject.Data.DataModels.Stub", b =>
    {
        b.HasOne("SampleProject.Data.DataModels.Transaction")
            .WithMany("Stubs")
            .HasForeignKey("TransactionId2");
    });

I'm obviously missing something but can't figure out what it is. I want to be able to use inheritance hierarchy with TPH approach and a single foreign key field from the base class table (TransactionItem) to the parent (Transaction). I appreciate any help that you can provide.

Thanks!

Answer 1

As pointed out by Ivan Stoev, I created this problem by having the parent object (Transaction in this case) implement read-only convenience properties for lists of my derived class types.

Because Transaction declared properties like below, the ef migration was creating new relationships/foreign keys for them.

public List<Check> Checks
{
    get
    {
        return this.TransactionItems?.OfType<Check>().ToList();
    }
}

I've since implemented these as methods so as not to conflict with the conventions of ef.

Many thanks to Ivan Stoev for prompting this discovery!