c# - Using both many-to-many and one-to-many to same entity

Question

I have a many-to-many association in EF Code-First (as explained in this question), and I want to use a one-to-many to the same entity as well. The problem is EF does not produce the right database scheme. Code:

public class A
{
  public int Id { get; set; }
  public string Name { get; set; }
  public virtual ICollection<B> ObjectsOfB { get; set; }
}

public class B
{
  public int Id { get; set; }
  public virtual A ObjectA { get; set; }
  public virtual ICollection<A> OtherObjectsOfA { get; set; }
}

When I remove the ObjectA property of class B the many-to-many association is generated correctly. When generated incorrectly, entity B gets 2 foreign keys to A, and entity A gets 1 foreign key to B (like a many-to-one relation).

Answer 1

If you have more than one navigation property refering to the same entity EF does not know where the inverse navigation property on the other entity belongs to. In your example: Does A.ObjectsOfB refer to B.ObjectA or to B.OtherObjectsOfA? Both would be possible and a valid model.

Now, EF does not throw an exception like "cannot determine relationships unambiguously" or something. Instead it decides that B.ObjectA refers to a third endpoint in B which is not exposed as navigation property in the model. This creates the first foreign key in table B. The two navigation properties in B refer to two endpoints in A which are also not exposed in the model: B.ObjectA creats the second foreign key in table B and B.OtherObjectsOfA creates a foreign key in table A.

To fix this you must specify the relationships explicitely.

Option one (the easiest way) is to use the InverseProperty attribute:

public class A
{
    public int Id { get; set; }
    public string Name { get; set; }
    [InverseProperty("OtherObjectsOfA")]
    public virtual ICollection<B> ObjectsOfB { get; set; }
}

This defines that A.ObjectsOfB is part of a many-to-many relation to B.OtherObjectsOfA.

The other option is to define the relationships completely in Fluent API:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<A>()
        .HasMany(a => a.ObjectsOfB)
        .WithMany(b => b.OtherObjectsOfA)
        .Map(x =>
        {
            x.MapLeftKey("AId");
            x.MapRightKey("BId");
            x.ToTable("ABs");
        });

    modelBuilder.Entity<B>()
        .HasRequired(b => b.ObjectA)  // or HasOptional
        .WithMany()
        .WillCascadeOnDelete(false);  // not sure if necessary, you can try it
                                      // without if you want cascading delete
}