Question

I have something that should be really simple, but it’s killing me.

l = LineItem.first
#<LineItem id: 5, product_id: 1, quantity: 1, price: #<BigDecimal:7f7fdb51a3f8,'0.999E3',9(18)>, cart_id: 5, discount_percentage: 10, discount_amount: nil, discount_active: true, created_at: "2012-01-12 16:17:41", updated_at: "2012-01-12 16:17:41">

I have

l.discount_percentage.blank?
=> false

So, I have the following method:

  def total_price
    discount_amount = 0 if discount_amount.blank?
    discount_percentage = 0 if discount_percentage.blank?

    discounted_amount_from_percent = price*(discount_percentage.to_f/100)

    applicable_discount = [discount_amount,discounted_amount_from_percent].max

    return (price-applicable_discount)
  end

But when I do this:

l.total_price

Instead of returning 899, it returns 999 (meaning that the if discount_percentage.blank? didn’t work at all!)

Or the syntax WHATEVER_HERE if true/false only work in the View on Rails??

Answer 1

Here lays the problem:

discount_amount = 0 if discount_amount.blank?
discount_percentage = 0 if discount_percentage.blank?

Ruby “sees” variables from top to bottom and from left to right, so in that line he first sees a local variable (discount_amount =) so he decides this discount_amount thing in discount_mount.blank? is that same local variable (and not the instance method. You think the variable is not defined yet, but Ruby has already spotted it). Not having any value yet, discount_amountit set to default value nil, so nil.blank? succeeds and the assignment discount_percentage = 0 is made. Ditto for discount_percentage. Here’s a demo snippet:

class ExampleClass 
  def run
    x = "it works as expected" if x == "x"
    x
  end

  def run2
    if x == "x"
      x = "it works as expected" 
    end
    x
  end

  def run3
    xy = "it works as expected" if x == "x"
    xy
  end

  def x; "x"; end
end

p ExampleClass.new.run #=> nil
p ExampleClass.new.run2 #=> "it works as expected"
p ExampleClass.new.run3 #=> "it works as expected"

Step 1: don’t use the same names for local variables and instance methods. That’s usually a bad idea anyway because you lose track of which one you are using, but in this case it has really bitten you.

Step 2: Do not write imperative code when you’re doing math calculations! Really, maths (9X % of the things you do in a typical application, (10-X)% being unavoidable side-effects) play well with expressions, not with statements. I’d write:

def total_price
  final_discount_amount = discount_amount || 0
  final_discount_percentage = discount_percentage || 0
  discounted_amount_from_percent = price * (final_discount_percentage.to_f/100)
  applicable_discount = [final_discount_amount, discounted_amount_from_percent].max
  price - applicable_discount
end