Having a decrement (-=) function stop at 0 in Ruby

Question

I have the following line of code which very simply decrements a Shareholders number of stocks (integer in the DB) when they sell them to someone else. This works well and as expected.

@selling_shareholder.update_attribute(:number_of_stocks, @selling_shareholder.number_of_stocks -= @transaction.number_of_stocks)

Very simply what I'd like to do is have the decrement function stop when the number_of_stocks hits 0 ie having a negative number should not be possible.

I guess I could use a simple unless @selling_shareholder.number_of_stocks > 0 at the end of the line, but I'm wondering if that will actually work without using a loop?

Answer 1

I would suggest creating a method in your model called something like #decrement_stock that would handle that logic. You can set all kind of behaviors that should be expected, like raising exceptions. That approach follows the " tell don't ask " principle. It is also a simple unit of work performing a single task, making it easy to test with your favorite unit testing framework.

As engineersmnky suggested, simply adding validation on the model is also a good solution if you want the controller to handle errors.