的（感谢约翰语法修复）的

所以我想通了。这是你必须做的：

So I figured it out. This is what you have to do:

db.Models.Order order = DB.Models.Order.CreateOrder( apple );
order.Customer = (from c in db.Customer where c.Id == custId select c).First();
db.SaveChanges();

我希望帮助的人。

I hope that helps people.