Multiple one-to-one relationship with two independent tables and one dependent

Question

I have read a lot of related questions about this topic but none of them seemed to address my problem, so please bear with me. I am new to EF and trying to establish the following relationship, in ASP .NET MVC, using EF6:

I need to have two permanent tables, Drivers and Cars. I now need to create a relationship between these tables when a Driver is associated to a Car. But one Driver can only be assigned to one Car.

A Driver may not always be associated to a Car and vice-versa and I want to maintain both tables even if there isn't always an association between them, so that is why I believe I need to have an additional table exclusively to make this connection. Which I think will create a 1:1:1 relationship between these classes.

Below is the model for my POCO classes.

Models

public class Driver
{
    public int DriverID { get; set; }
    public string Name { get; set; }
    //other additional fields

    public DriverCar DriverCar { get; set; }
}

public class Car
{
    public int CarID { get; set; }
    public string Brand { get; set; }
    //other additional fields

    public DriverCar DriverCar { get; set; }
}

public class DriverCar
{
    public int DriverCarID { get; set; }

    public int DriverID { get; set; }
    public Driver Driver { get; set; }

    public int CarID { get; set; }
    public Car Car { get; set; }
 }

I have tried configuration the relationships using Fluent API but I believe I am doing it completly wrong since I have got errors such as:

Introducing FOREIGN KEY constraint 'FK_dbo.DriverCar_dbo.Car_CarId' on table 'DriverCar' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints. Could not create constraint or index. See previous errors.

Fluent Api

modelBuilder.Entity<DriverCar>()
                        .HasRequired(a => a.Driver)
                        .WithOptional(s => s.DriverCar)
                        .WillCascadeOnDelete(false);

modelBuilder.Entity<DriverCar>()
                        .HasRequired(a => a.Car)
                        .WithOptional(s => s.DriverCar)
                        .WillCascadeOnDelete(false);

I am really not sure if I am missing something or if there is some better approach to handle this situation and I would appreciate so much if someone can give me some feedback on how to solve this.

---

Update

Just found an interesting answer here: Is it possible to capture a 0..1 to 0..1 relationship in Entity Framework? Which I believe is exactly what I want: a 0..1 to 0..1 relationship. But all the mentioned options seem too complex and I'm not quite sure which one is the best or how to even correctly implement them.

Are these type of relationships supposed to be so hard to implement in EF? For example, I tried Option 1 but it created a 0..1 to many relationship from both tables - Driver to Car and Car to Driver. How am I suppose to create an unique association between them then?

Answer 1

Try this for your models. Virtual enables lazy loading and is advised for navigation properties. DataAnnotations showing the Foreign Keys (or use fluent) to be sure each relationship is using the correct key.

public class Driver
{
    public int DriverID { get; set; }
    public string Name { get; set; }
    //other additional fields

    public DriverCar? DriverCar { get; set; }
}

public class Car
{
    public int CarID { get; set; }
    public string Brand { get; set; }
    //other additional fields

    public DriverCar? DriverCar { get; set; }
}

public class DriverCar
{
    public int DriverCarID { get; set; }

    [ForeignKey("Driver")]
    public int DriverID { get; set; }
    public Driver Driver { get; set; }

    [ForeignKey("Car")]
    public int CarID { get; set; }
    public Car Car { get; set; }
 }

modelBuilder.Entity<Driver>()
                        .HasOptional(a => a.DriverCar)
                        .WithRequired(s => s.Driver)
                        .WillCascadeOnDelete(false);

modelBuilder.Entity<Car>()
                        .HasOptional(a => a.DriverCar)
                        .WithRequired(s => s.Car)
                        .WillCascadeOnDelete(false);

Note: Changed to Data Annotations for Foreign Keys. Inverted fluent statements. Fixed Driver to Car in second relationship.

Answer 2

Here is a simple way to create a one to zero. Note that I'm a fan of keeping the Id of all tables as just Id, not CarId etc, just my style. This is just a console app so once you add the EF nuget you could just copy/paste.

But the below code works with .net framework 4.6 and EF6.2 It creates the following tables

Car

• Id (PK, int, not null)
• Driver_Id (FK, int, null)

Driver

• Id (PK, int, not null)

Under this schema a Car can have only one driver. A driver may still drive multiple cars though. I'm not sure if that's an issue for you or not.

    using System.Data.Entity;

    namespace EFTest
    {
        class Program
        {
            static void Main(string[] args)
            {
                var connectionString = "<your connection string>";
                var context = new DatabaseContext(connectionString);

                var car = new Car();
                var driver = new Driver();

                context.Cars.Add(car);
                context.Drivers.Add(driver);
                car.Driver = driver;

                context.SaveChanges();

            }
        }

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

        public class DatabaseContext : DbContext, IDatabaseContext
        {
            public DbSet<Car> Cars { get; set; }
            public DbSet<Driver> Drivers { get; set; }

            public DatabaseContext(string connectionString) : base(connectionString){ }

            protected override void OnModelCreating(DbModelBuilder modelBuilder)
            {
                modelBuilder.Entity<Car>()
                    .HasKey(n => n.Id)
                    .HasOptional(n => n.Driver);

                modelBuilder.Entity<Driver>()
                    .HasKey(n => n.Id);
            }

        }
    }

But if you REALLY wanted to enforce the constraint of only one mapping per car and driver, you could do it with the code below. Note that when you have the joining entity, you don't put it's Id anywhere on the joined entities.

using System.Data.Entity;

namespace EFTest
{
class Program
{
    static void Main(string[] args)
    {
        var connectionString = "your connection string";
        var context = new DatabaseContext(connectionString);

        //Create a car, a driver, and assign them
        var car = new Car();
        var driver = new Driver();
        context.Cars.Add(car);
        context.Drivers.Add(driver);
        context.SaveChanges();
        var assignment = new DriverAssignment() { Car_id = car.Id, Driver_Id = driver.Id };
        context.DriverAssignments.Add(assignment);
        context.SaveChanges();

        //Create a new car and a new assignment
        var dupCar = new Car();
        context.Cars.Add(dupCar);
        context.SaveChanges();
        var dupAssignment = new DriverAssignment() { Car_id = dupCar.Id, Driver_Id = driver.Id };
        context.DriverAssignments.Add(dupAssignment);

        //This will throw an exception because it will violate the unique index for driver.  It would work the same for car.
        context.SaveChanges();

    }
}

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

public class DriverAssignment
{
    public int Car_id { get; set; }

    public int Driver_Id { get; set; }
}


public class DatabaseContext : DbContext, IDatabaseContext
{
    public DbSet<Car> Cars { get; set; }
    public DbSet<Driver> Drivers { get; set; }

    public DbSet<DriverAssignment> DriverAssignments { get; set; }

    public DatabaseContext(string connectionString) : base(connectionString) { }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Car>().HasKey(n => n.Id);
        modelBuilder.Entity<Driver>().HasKey(n => n.Id);
        modelBuilder.Entity<DriverAssignment>().HasKey(n => new { n.Car_id, n.Driver_Id });
        modelBuilder.Entity<DriverAssignment>().HasIndex(n => n.Car_id).IsUnique();
        modelBuilder.Entity<DriverAssignment>().HasIndex(n => n.Driver_Id).IsUnique();
    }

}

}