Unable to determine the principal end of an association

Question

In this kind of association I get the error "Unable to determine the principal end of an association between the types 'Foo' and 'Bar'. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations."

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

    public int? MainBarId { get; set; }
    public virtual Bar MainBar { get; set; }

    [InverseProperty("Foo")]
    public virtual ICollection<Bar> Bars { get; set; }
}

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

    public int FooId { get; set; }
    public virtual Foo Foo { get; set; }

    public int? OldFooId { get; set; }
    public virtual Foo OldFoo { get; set; }
}

Here the Foo has a collection of Bars and could have a main Bar (MainBar). The Bar always was associated with a Foo but could have a reference to another Foo (OldFoo).

1. How to map this in EF with Data Annotations?
2. If not possible with Data Annotations, how to do this with fluent?

Answer 1

By [InverseProperty("Foo")] you tell EF that Bar.Foo and Foo.Bars are paired properties in a one to many association, so that's clear.

Then there are Foo.MainBar and Bar.OldFoo. EF does not know how these are related. They could be paired in a one-to-one association, they can be independent, i.e. with a "many" multiplicity on the other side. So you have to tell EF.

I assume that the properties are independent, i.e. that a Bar can have an OldFoo without the requirement that this Bar is Foo's MainBar at the same time. Then it is enough to give EF information about one of the properties:

modelBuilder.Entity<Bar>().HasOptional(f => f.OldFoo).WithMany()
    .HasForeignKey(f => f.OldFooId);

or

modelBuilder.Entity<Foo>().HasOptional(f => f.MainBar)
    .WithRequired(b => b.OldFoo)

As there are no inverse properties paired with these "one" ends of the associations you can't do this with data annotations (there are no properties to adorn with attributes).