When you add one callack in two separate `set_callback` calls - it is
only called once.
When you do it in one `set_callback` call - it is called twice.
This violates the principle of least astonishment for me. Duplicating
callback is usually an error. There is a correct and obvious way to do
anything without this "feature".
If you want to do
before_save :clear_balance, :calculate_tax, :clear_balance
or whatever, you should better do
And this even opens gates for some advanced refactorings, unlike the
My assumptions are:
- Principle of least astonishment is violated, when callbacks are either
prevented from duplication, or not.
- Duplicating callbacks is usually an error. When it is intentional -
it's a smell of a bad design and can be approached without abusing
My suggestion is: do not allow duplicating callbacks in one callback
call, like it is not allowed in separate callbacks call.