EF Code First: Migrations removing needed columns and adding unwanted ones - ef-code-first

Question

Note: there is a "better" iteration of this question here:
EF Code First migrations: Table Per Hierarchy Bug
I am trying to figure out the exact steps need to take an existing database and overlay entity framework code first with a Table Per Hierarchy structure. However no matter what I do, the end result migration is "wrong".
Here is the sample database, exact steps and output.
Existing database structure
CREATE TABLE [dbo].[Categories](
[Id] [int] IDENTITY(1,1) NOT NULL Primary Key,
[Name] [nvarchar](100) NOT NULL,
)
GO
CREATE TABLE [dbo].[A](
[Id] [int] IDENTITY(1,1) NOT NULL Primary Key,
[Discriminator] [nvarchar](20) NOT NULL,
[Description] [nvarchar](100) NOT NULL,
[CategoryId] [int] NULL
)GO
ALTER TABLE [dbo].[A]
WITH CHECK ADD CONSTRAINT [FK_dbo.A_dbo.Categories_CategoryId]
FOREIGN KEY([CategoryId])
REFERENCES [dbo].[Categories] ([Id])
GO
ALTER TABLE [dbo].[A]
CHECK CONSTRAINT [FK_dbo.A_dbo.Categories_CategoryId]
GO
"Reverse Engineer Code First" generates the following:
public partial class EntityContext : DbContext {
static EntityContext() {
Database.SetInitializer<EntityContext>(null);
}
public EntityContext()
: base("Name=Test47Context") {
}
public DbSet<A> A { get; set; }
public DbSet<Category> Categories { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder) {
modelBuilder.Configurations.Add(new AMap());
modelBuilder.Configurations.Add(new CategoryMap());
}
}
public partial class A {
public int Id { get; set; }
public string Discriminator { get; set; }
public string Description { get; set; }
public Nullable<int> CategoryId { get; set; }
public virtual Category Category { get; set; }
}
public partial class Category {
public Category() {
this.A = new List<A>();
}
public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection<A> A { get; set; }
}
public class AMap : EntityTypeConfiguration<A> {
public AMap() {
// Primary Key
this.HasKey(t => t.Id);
// Properties
this.Property(t => t.Discriminator)
.IsRequired()
.HasMaxLength(20);
this.Property(t => t.Description)
.IsRequired()
.HasMaxLength(100);
// Table & Column Mappings
this.ToTable("A");
this.Property(t => t.Id).HasColumnName("Id");
this.Property(t => t.Discriminator).HasColumnName("Discriminator");
this.Property(t => t.Description).HasColumnName("Description");
this.Property(t => t.CategoryId).HasColumnName("CategoryId");
// Relationships
this.HasOptional(t => t.Category)
.WithMany(t => t.A)
.HasForeignKey(d => d.CategoryId);
}
}
public class CategoryMap : EntityTypeConfiguration<Category> {
public CategoryMap() {
// Primary Key
this.HasKey(t => t.Id);
// Properties
this.Property(t => t.Name)
.IsRequired()
.HasMaxLength(100);
// Table & Column Mappings
this.ToTable("Categories");
this.Property(t => t.Id).HasColumnName("Id");
this.Property(t => t.Name).HasColumnName("Name");
}
}
Implement TPH, by
Adding Derived.cs inherits from A.cs
Moving Category and CategoryId properties from A.cs to Derived.cs
Remove Discriminator property from A.cs
In AMap.cs remove Discriminator, CategoryId and Category Foreign Key mapping.
Adding TPH mapping to OnModelCreating
public partial class EntityContext : DbContext {
static EntityContext() {
Database.SetInitializer<EntityContext>(null);
}
public EntityContext()
: base("Name=Test47Context") {
}
public DbSet<A> A { get; set; }
public DbSet<Category> Categories { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder) {
modelBuilder.Configurations.Add(new AMap());
modelBuilder.Configurations.Add(new CategoryMap());
modelBuilder.Entity<A>()
.Map<Derived>(x => x.Requires("Discriminator").HasValue("Derived"));
}
}
public partial class A {
public int Id { get; set; }
public string Description { get; set; }
}
public class Derived : A {
public Nullable<int> CategoryId { get; set; }
public virtual Category Category { get; set; }
}
public partial class Category {
public Category() {
this.A = new List<A>();
}
public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection<A> A { get; set; }
}
public class AMap : EntityTypeConfiguration<A> {
public AMap() {
// Primary Key
this.HasKey(t => t.Id);
this.Property(t => t.Description)
.IsRequired()
.HasMaxLength(100);
// Table & Column Mappings
this.ToTable("A");
this.Property(t => t.Id).HasColumnName("Id");
this.Property(t => t.Description).HasColumnName("Description");
}
}
public class CategoryMap : EntityTypeConfiguration<Category> {
public CategoryMap() {
// Primary Key
this.HasKey(t => t.Id);
// Properties
this.Property(t => t.Name)
.IsRequired()
.HasMaxLength(100);
// Table & Column Mappings
this.ToTable("Categories");
this.Property(t => t.Id).HasColumnName("Id");
this.Property(t => t.Name).HasColumnName("Name");
}
}
"Add-migration TPH" generates the following
public partial class TPH : DbMigration
{
public override void Up()
{
AddColumn("dbo.A", "Category_Id", c => c.Int());
AddForeignKey("dbo.A", "Category_Id", "dbo.Categories", "Id");
CreateIndex("dbo.A", "Category_Id");
DropColumn("dbo.A", "Discriminator");
}
public override void Down()
{
AddColumn("dbo.A", "Discriminator", c => c.String(nullable: false, maxLength: 20));
DropIndex("dbo.A", new[] { "Category_Id" });
DropForeignKey("dbo.A", "Category_Id", "dbo.Categories");
DropColumn("dbo.A", "Category_Id");
}
}
Why does it drop the Discriminator column instead of just altering it to nvarchar(128)?
Why doesn't just use the existing CategoryId column instead of adding Category_Id?