the member OrderKeyID of OrderKey class (and other class) are

DataWebKeyAttribute was initially used in ASP.Net 3.5 extensions, after VS 2008 SP1 was released DataServiceKeyAttribute replaced it but it is wrongly used in the example above .
The code should be something like:

[DataServiceKey("OrderKey")]
public class Order
{
...
//no attribute here
public int OrderKey
{
get { return _ID; }
}
...
}