Entity Framework with optional child navigation property creates extra columns - entity-framework

Question

I've just noticed a rather strange behaviour with how Entity Framework creates a Code-First DB when you have a model with a parent that has a list of children, but also an optional navigation property to one particular child: I end up with an additional nullable foreign key column on the child that I'm not expecting. Can anyone please explain whether this column is actually necessary? And for that matter, can anyone suggest a better way of indicating that a particular child is Selected/Active.
To elaborate:
Given this model:
public class Parent
{
public int Id { get; set; }
public virtual List<Child> Children { get; set; }
// Optional navigation property to one of the child objects.
public int? ActiveChildId { get; set; }
public virtual Child ActiveChild { get; set; }
}
public class Child
{
public int Id { get; set; }
public int ParentId { get; set; }
public virtual Parent Parent { get; set; }
}
I end up with the following DB:
CREATE TABLE [dbo].[Parents](
[Id] [int] IDENTITY(1,1) NOT NULL,
[ActiveChildId] [int] NULL,
CONSTRAINT [PK_dbo.Parents] PRIMARY KEY CLUSTERED
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[Parents] WITH CHECK ADD CONSTRAINT [FK_dbo.Parents_dbo.Children_ActiveChildId] FOREIGN KEY([ActiveChildId])
REFERENCES [dbo].[Children] ([Id])
GO
ALTER TABLE [dbo].[Parents] CHECK CONSTRAINT [FK_dbo.Parents_dbo.Children_ActiveChildId]
GO
CREATE TABLE [dbo].[Children](
[Id] [int] IDENTITY(1,1) NOT NULL,
[ParentId] [int] NOT NULL,
[Parent_Id] [int] NULL,
CONSTRAINT [PK_dbo.Children] PRIMARY KEY CLUSTERED
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[Children] WITH CHECK ADD CONSTRAINT [FK_dbo.Children_dbo.Parents_Parent_Id] FOREIGN KEY([Parent_Id])
REFERENCES [dbo].[Parents] ([Id])
GO
ALTER TABLE [dbo].[Children] CHECK CONSTRAINT [FK_dbo.Children_dbo.Parents_Parent_Id]
GO
ALTER TABLE [dbo].[Children] WITH CHECK ADD CONSTRAINT [FK_dbo.Children_dbo.Parents_ParentId] FOREIGN KEY([ParentId])
REFERENCES [dbo].[Parents] ([Id])
GO
ALTER TABLE [dbo].[Children] CHECK CONSTRAINT [FK_dbo.Children_dbo.Parents_ParentId]
GO
i.e. there is both a ParentId (NOT NULL) column on the child AND a Parent_Id (NULL) column on the child.
It seems to me that since we've already got a 1:N foreign key relationship Parent to Child, then by adding a one-way 1:[0 or 1] Parent to Child relationship it shouldn't create another foreign key column on the child.

Answer 1

Add the InverseProperty attribute:
public class Child
{
public int Id { get; set; }
public int ParentId { get; set; }
[InverseProperty( "Children" )]
public virtual Parent Parent { get; set; }
}
Or map the relationship via Fluent API and specify the FK as ParentId:
modelBuilder.Entity<Parent>()
.HasMany( p => p.Children )
.WithRequired( c => c.Parent )
.HasForeignKey( c => c.ParentId );