How do you model 1x0 relations. 1 (optional one-to-one) in the Entity Framework Code First?

Question

When an Entity CAN have one (and only one) instance of another Entity, but the second entity must have the first, as the Entities modeling is done?

Answer 1

Suppose two models: Drivers (drivers) and Vehicles (vehicles). Also suppose a driver doesn’t necessarily have a vehicle, so you can create a driver first and then assign a vehicle to him:

---

Models:

public class Driver
{
    [Key]
    public Guid DriverId { get; set; }
    public Guid? VehicleId { get; set; }

    public virtual Vehicle Vehicle { get; set; }
}

public class Vehicle
{
    [Key]
    public Guid VehicleId { get; set; }
    public Guid DriverId { get; set; }

    public virtual Driver Driver { get; set; }
}

---

Controllers:

public class DriversController : Controller
{
    private TesteMarotoContext context = new TesteMarotoContext();

    //
    // GET: /Drivers/

    public ViewResult Index()
    {
        return View(context.Drivers.Include(driver => driver.Vehicle).ToList());
    }

    //
    // GET: /Drivers/Details/5

    public ViewResult Details(System.Guid id)
    {
        Driver driver = context.Drivers.Single(x => x.DriverId == id);
        return View(driver);
    }

    //
    // GET: /Drivers/Create

    public ActionResult Create()
    {
        ViewBag.PossibleVehicles = context.Vehicles;
        return View();
    } 

    //
    // POST: /Drivers/Create

    [HttpPost]
    public ActionResult Create(Driver driver)
    {
        if (ModelState.IsValid)
        {
            driver.DriverId = Guid.NewGuid();
            context.Drivers.Add(driver);
            context.SaveChanges();

            if (driver.VehicleId != null)
            {
                var vehicle = context.Vehicles.Single(v => v.VehicleId == driver.VehicleId);
                vehicle.DriverId = driver.DriverId;
                context.Entry(vehicle).State = EntityState.Modified;
                context.SaveChanges();
            }

            return RedirectToAction("Index");  
        }

        ViewBag.PossibleVehicles = context.Vehicles;
        return View(driver);
    }

    //
    // GET: /Drivers/Edit/5

    public ActionResult Edit(System.Guid id)
    {
        Driver driver = context.Drivers.Single(x => x.DriverId == id);
        ViewBag.PossibleVehicles = context.Vehicles;
        return View(driver);
    }

    //
    // POST: /Drivers/Edit/5

    [HttpPost]
    public ActionResult Edit(Driver driver)
    {
        if (ModelState.IsValid)
        {
            context.Entry(driver).State = EntityState.Modified;
            context.SaveChanges();

            if (driver.VehicleId != null)
            {
                var vehicle = context.Vehicles.Single(v => v.VehicleId == driver.VehicleId);
                vehicle.DriverId = driver.DriverId;
                context.Entry(vehicle).State = EntityState.Modified;
                context.SaveChanges();
            }

            return RedirectToAction("Index");
        }
        ViewBag.PossibleVehicles = context.Vehicles;
        return View(driver);
    }

    //
    // GET: /Drivers/Delete/5

    public ActionResult Delete(System.Guid id)
    {
        Driver driver = context.Drivers.Single(x => x.DriverId == id);
        return View(driver);
    }

    //
    // POST: /Drivers/Delete/5

    [HttpPost, ActionName("Delete")]
    public ActionResult DeleteConfirmed(System.Guid id)
    {
        Driver driver = context.Drivers.Single(x => x.DriverId == id);
        context.Drivers.Remove(driver);
        context.SaveChanges();
        return RedirectToAction("Index");
    }

    protected override void Dispose(bool disposing)
    {
        if (disposing) {
            context.Dispose();
        }
        base.Dispose(disposing);
    }
}

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Note that you need to set Id’s manually (by contrast).