Insert/update multiple objects
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; }
}

<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>

[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);
    }

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();