How to Create a One-to-One Relationship Using a Non-Primary Key (EF First Code)

You cannot create a one-to-one (or one-to-one) relationship between two EF SQL entities / tables using a non-primary key property / column. You should be using primary key columns to achieve this kind of relationship with EF.

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

    public Guid CarGuid { get; set; }

    public string Name { get; set; }

    public virtual CarDetails CarDetails { get; set; }
}

public class CarDetails
{
    [Key, ForeignKey("Car")]
    public int Id { get; set; }

    public Guid CarGuid { get; set; }

    public string Color { get; set; }

    public int Weight { get; set; }

    public virtual Car Car { get; set; }
}

      

        
        
        
      

    

With the above relationship you will get Car

as principal and as CarDetails

dependent. By having CarDetails.Id

both the primary key CarDetails

and the foreign key referencing Car.Id

, you can ensure that each Car

has at most 1 CarDetails

.

However, this is not technically a relationship 1<->1

... it is a relationship 1<->0..1

. This type of model will allow you to create objects Car

without corresponding objects CarDetils

.

If you need something closer to a relationship 1<->1

, I believe the only way to do it is with a free modeler:

public class CarsDbContext : DbContext
{
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Car>();
        modelBuilder.Entity<CarDetails>()
            .HasRequired(x => x.Car).WithRequiredDependent(x => x.CarDetails)
        ;

        base.OnModelCreating(modelBuilder);
    }
}

      

        
        
        
      

    

In the above definition, you cannot save Car

unless you save the corresponding CarDetails

one within the same SaveChanges operation. This will be as close to as possible 1<->1

as the object accepts. Similarly, you can achieve the same on the other side of the relationship:

public class CarsDbContext : DbContext
{
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Car>()
            .HasRequired(x => x.CarDetails).WithRequiredPrincipal(x => x.Car)
        ;
        modelBuilder.Entity<CarDetails>();

        base.OnModelCreating(modelBuilder);
    }
}

      

        
        
        
      

    

Link: http://www.entityframeworktutorial.net/code-first/configure-one-to-one-relationship-in-code-first.aspx