Entity Framework HasForeignKey error with 1:1 Relationship - entity-framework

Question

When setting up a one-to-many relationship I can do this and it works:
EntityTypeBuilder<BrandOwner> brandOwner = X
brandOwner.HasMany(a => a.Brands).WithOne(b => b.Owner).HasForeignKey("OwnerId");
In a 1:1 relationship I can use this syntax and it works (observe generic HasForeignKey)
person.HasOne(a => a.SinCard).WithOne(b => b.Owner).HasForeignKey<SocialInsuranceCard>(c => c.OwnerId);
But this gives an error:
person.HasOne(a => a.SinCard).WithOne(b => b.Owner).HasForeignKey("OwnerId");
The error is:
InvalidOperationException: You are configuring a relationship between 'Person' and 'SocialInsuranceCard' but have specified a foreign key on 'OwnerId'. The foreign key must be defined on a type that is part of the relationship.
Sample classes:
public class BrandOwner
{
public int Id { get; set; }
public virtual List<Brand> Brands { get; set; }
}
public class Brand
{
public int Id { get; set; }
public int OwnerId { get; set; } // I'm aware of the "BrandOwnerId" convention
public virtual BrandOwner Owner { get; set; }
}
pubic class Person
{
public int Id { get; set; }
public virtual SocialInsuranceCard SinCard { get; set; }
}
pubic class SocialInsuranceCard
{
public int Id { get; set; }
public int OwnerId; // I know I could get auto mapping by using "PersonId"
public virtual Person Owner { get; set; }
}
Why in the 1:1 relationship does using the non-generic HasForeignKey give an error?

Answer 1

The structure you have is not 1-to-1, it is 1-to-many. A 1-to-1 is generally set up to share the same ID:
public class Car
{
public int CarId { get; set; }
public virtual Seat seat { get; set; }
}
public class Seat
{
public int CarId { get; set; } // PK and FK to Car
public virtual Car car { get; set; }
}
This assumes 1 car has 1 seat. If you want to identify something like a Primary Seat within a car that can have multiple seats, you might want something like:
public class Car
{
public int CarId { get; set; }
public virtual ICollection<Seat> Seats { get; set; }
public virtual Seat PrimarySeat { get; set; }
}
public class Seat
{
public int SeatId { get; set; } // PK
public int CarId { get; set; } // FK to Car 1-to-Many
public virtual Car car { get; set; }
}
This relationship would need a Seat ID on Car:
public class Car
{
// ...
public int PrimarySeatId { get; set; } // FK for Primary Seat
}
Setting this up may be possible, but would require explicit mapping for all car and seat relationships rather than leaving it to convention. Car will explicitly need to know how to resolve its Seats collection (which use the CarId on Seat) and to resolve the PrimarySeat instance. (many-to-1) It's not a 1-to-1 specifically because there is nothing stopping multiple cars from specifying the same SeatID as the primary. Additionally, there is nothing enforcing that the PrimarySeat even belongs to the Car's Seats collection either. Any such restrictions and rules would need to be enforced at the application level, possibly supported by a health check query run against the database daily to report any mismatches. (Cars with primary seats who's carId does not match)