Model configuration: Ignoring base types can leave behind unnecessary discriminator column

[dealdiane]

Suppose we have this model:

public class Cart
{
    public int Id { get; set; }
}

public class CartItem
{
    public Cart Cart { get; set; }

    public int Id { get; set; }
}

public class ExtendedCart : Cart
{
}

public class ExtendedCartItem : CartItem
{
    public ExtendedCart InternalCart { get; set; }
}

and is configured like this:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);

    modelBuilder.Ignore<CartItem>();
    modelBuilder.Entity<ExtendedCartItem>()
        .Ignore(i => i.Cart);

    modelBuilder.Ignore<Cart>();
    modelBuilder.Entity<ExtendedCart>();
}

Running migrations will produce this:

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.CreateTable(
        name: "ExtendedCart",
        columns: table => new
        {
            Id = table.Column<int>(nullable: false)
                .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn),
            Discriminator = table.Column<string>(nullable: true)
        },
        constraints: table =>
        {
            table.PrimaryKey("PK_ExtendedCart", x => x.Id);
        });

    migrationBuilder.CreateTable(
        name: "ExtendedCartItem",
        columns: table => new
        {
            Id = table.Column<int>(nullable: false)
                .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn),
            InternalCartId = table.Column<int>(nullable: true)
        },
        constraints: table =>
        {
            table.PrimaryKey("PK_ExtendedCartItem", x => x.Id);
            table.ForeignKey(
                name: "FK_ExtendedCartItem_ExtendedCart_InternalCartId",
                column: x => x.InternalCartId,
                principalTable: "ExtendedCart",
                principalColumn: "Id",
                onDelete: ReferentialAction.Restrict);
        });

    migrationBuilder.CreateIndex(
        name: "IX_ExtendedCartItem_InternalCartId",
        table: "ExtendedCartItem",
        column: "InternalCartId");
}

Notice the Discriminator column in the ExtendedCart table. I'm ignoring the Cart type and Cart property of the CartItem type. The Cart type is referenced nowhere. Do we really still need a discriminator column here?

This used to work fine without the need for a discriminator column before 1.1.0.

Further technical details

EF Core version: 1.1.0
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
IDE: Visual Studio 2015

[smitpatel]

@AndriySvyryd - Can I poach this?
Solution: Remove discriminator column when no base type & no directly derived type?

[AndriySvyryd]

@smitpatel You can poach all of my issues, no need to ask upfront.
The proposed solution is correct.

[smitpatel]

This reproduces in 1.1.1 release but works correctly in nightly builds. This seem to be related to #8151 In #8151, Ignored entity type was still base type, throwing exception while setting key in model builder but later gets cleaned up. In this one, perhaps the base type is still set so that DiscriminatorConvention did not remove the discriminator column but the base type was removed later, leaving the extra column behind.