How to design class dependency trying to avoid Law of Demeter

Question

Ok,I´ve searched and couldn´t find a suitable solution for my problem, I am redesigning a part of our point of sale system. Let´s suppose we have the following classes:

TWorkShift = class
   Date: TDateTime;
   fTotalSold: Currency;
   fSales: TList<TSale>; 
public
   property TotalSold: Currency read fTotalSold write fTotalSold;
   property Sales: Currency read fSales write fSales;
end;

TSale = class
    fAmount: Currency;
    fWorkShift: TWorkShift;
public
    property Amount: Currency read fAmount write fAmount; 
    procedure Save;  
end;

Now, the problem I am facing is trying to come to the best idea without violating the Law of Demeter. What I am trying to accomplish is the following:

1. Every time a new TSale is saved I want to add it to the Sales list of the TWorkShift of the current user, and also I want to sum the amount of the sale to the "TotalSold" of the TWorkShift.

I´ve tried two different approaches:

Approach A:

// Let´s suppose we have a working shift with the ID 1 and gets loaded from database with: CurrentShift := TWorkShift.Create(1);

NewSale := TSale.Create;
NewSale.Amount:=100;
NewSale.Save;

CurrentShift.Sales.Add(NewSale);
CurrentShift.TotalSold := CurrentShift.TotalSold + NewSale.Amount;

The problem with this approach is that It´s difficult to test, because I want to encapsulate the logic of the sum in some of the classes or somewhere else (a new class maybe?).

Approach B:

My other approach is including that code inside the TSale class itself:

procedure TSale.Save;
begin
    SaveToDataBase; 

    fWorkShift.Sales.Add(Self);
    fWorkShift.TotalSold := fWorkShift.TotalSold + Self.Amount;
end;

This approach I think violates the Law of Demeter and doesn´t feel right to me.

I Want to find a "right way" to do it maximizing code simplicity and easy of maintenance in the future. So any suggestions would be appreciated.

Thanks

Answer 1

If you want to add a sale to TWorkShift, then you should have

TWorkShift.AddSale(aSale: TSale);
begin
  Sales.Add(aSale);
end;

In other words, TWorkShift should "ask" for the thing it needs.

Also, I don't see any reason that TSale would have a TWorkShift field. A Workshift has many sales, but why would a Sale have a WorkShift?