EF Code First, two navigation properties to same object

Question

I'm using EF Code First to query and create a database. One of my entities (relationship) has two navigation properties to the same entity (activity). My problem is that if I use EF to create the database schema it will create four foreign key columns and constraints instead of two.

Here are the relevant code parts:

activity class:

public class Activity {
    public virtual ICollection<Relationship> Successors { get; set; }
    public virtual ICollection<Relationship> Predecessors { get; set; }
}

relationship class:

public class Relationship {
    public virtual Activity Activity1 { get; set; }
    public int Activity1_ID { get; set; }
    public virtual Activity Activity2 { get; set; }
    public int Activity2_ID { get; set; }
}

Relationship mapping class:

this.HasRequired(t => t.Activity1)
  .WithMany(t => t.Predecessors)
  .HasForeignKey(m => m.Activity1_ID)
  .WillCascadeOnDelete(false);
this.HasRequired(t => t.Activity2)
  .WithMany(t => t.Successors)
  .HasForeignKey(m => m.Activity2_ID)
  .WillCascadeOnDelete(false);

Database structure:

Is there a way to prevent the creation of the last two columns?

Answer 1

This should create you only 2 foreign key columns.

public class Activity
{
    public int Id { set; get; }
    public virtual ICollection<Relationship> Successors { get; set; }
    public virtual ICollection<Relationship> Predecessors { get; set; }
}
public class Relationship
{
    public int Id { set; get; }
    public virtual Activity Activity1 { get; set; }
    public int Activity1_ID { get; set; }
    public virtual Activity Activity2 { get; set; }
    public int Activity2_ID { get; set; }
}

And the DbContext class where i am specifying the relationship/FK nature on my OnModelCreating.

public class MyDb: DbContext
{
    public MyDb():base("EfDbContext")
    {           
    }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {            
        modelBuilder.Entity<Relationship>()
            .HasRequired(f => f.Activity1)
            .WithMany(f => f.Predecessors)
            .HasForeignKey(g => g.Activity1_ID)
            .WillCascadeOnDelete(false);

        modelBuilder.Entity<Relationship>().
             HasRequired(f => f.Activity2)
            .WithMany(f => f.Successors)
            .HasForeignKey(g => g.Activity2_ID)
            .WillCascadeOnDelete(false);
    }
}