Insert/update multiple objects - entity-framework

Question

I have the following class:
public class Location
{
public int Id { get; set; }
public string Description { get; set; }
public string CarId { get; set; }
public Car Car { get; set; }
}
public class Car
{
public string Id { get; set; }
public string Color { get; set; }
}
And a view:
<div>#Html.LabelFor(location => location.Description)</div>
<div>#Html.EditorFor(location => location.Description)</div>
<div>#Html.LabelFor(location => location.Car.Id)</div>
<div>#Html.EditorFor(location => location.Car.Id)</div>
<div>#Html.LabelFor(location => location.Car.Color)</div>
<div>#Html.EditorFor(location => location.Car.Color)</div>
When i try this:
[HttpPost]
public ActionResult Create(Location location)
{
if (ModelState.IsValid)
{
Car car = db.Car.Find(location.Car.Id);
if (car != null)
db.Entry(car).CurrentValues.SetValues(location.Car);
else
db.Car.Add(location.Car);
db.Location.Add(locacao);
db.SaveChanges();
return RedirectToAction("Index");
}
return View(locacao);
}
it breaks in 'db.SaveChanges' cause it says 'Cannot insert duplicate key in object 'dbo.Car'.'
If I remove 'db.Location.Add(locacao);', so it works pretty fine for the car(insert and update), but no location is added in database.
How can I do to insert a new car when there is not car's id in database, update when there is, and insert a new location?

Answer 1

Add method always adds all entities in the object graph so both db.Car.Add(location.Car) and db.Location.Add(location) are inserting both location and car.
Try this:
db.Location.Add(locacation);
// You can also use another way to find if you are working with a new Car
// like location.Car.Id == 0
if (db.Car.Any(c => c.Id == location.Car.Id))
{
db.Entry(location.Car).State = EntityState.Modified;
}
db.SaveChanges();