[mattmanser]

It completely depends if you do this:

    public static decimal DiscountValue(Order order)

Or this:

    public static decimal DiscountValue(decimal orderValue, decimal orderDiscount)

Or even this, which would farm the work out to SQL:

    public static decimal DiscountValue(int orderId)

The first would require a mock, the 2nd wouldn't and the third isn't unit testable.

It's a bit of a contrived example, as you're more likely to do the third method if you're trying to get some sort of summary data about an order out which would mean you don't really need/want to load the whole order object.