Question

I have a CommercialDocument model which have a discount_amount attribute and a discount_amount_with_tax virtual attribute.

Here is how I defined this in my model :

def discount_amount_with_tax
  discount_amount * (1 + tax.rate / 100)
end

def discount_amount_with_tax=(amount)
  self.discount_amount = amount.to_f / (1 + tax.rate / 100)
end

In my form, a user can fill in both discount_amount and discount_amount_tax :

= f.label :discount_amount
= f.text_field :discount_amount
= f.text_field :discount_amount_with_tax

I want to give the priority to the discount_amount_with_tax field, which means that discount_amount must not be taken into account unless the other field is empty.

My problem is that if I put nothing in the discount_amount_with_tax field, and let’s say 10 in discount_amount, then discount_amount will be equal to 0, which is clearly not what I want.

How can I fix this ?

Any help would be greatly appreciated.

Answer 1

"".to_i
# => 0

A blank string converts to a zero integer. Therefore:

def discount_amount_with_tax=(amount)
  self.discount_amount = amount.to_f / (1 + tax.rate / 100)
end

# same as...

def discount_amount_with_tax=(0)
  self.discount_amount = 0 / (...)
end

# 0 / anything except zero = 0
# self.discount_amount = 0 no matter what

During mass-assignment, discount_amount_with_tax= is called. A blank form input is passed as an empty string, which Active Record then converts to an integer (zero). discount_amount_with_tax= sets discount_amount to zero regardless of discount_amount‘s previous value.

Easy way around this is to use a conditional:

def discount_amount_with_tax=(amount)
  self.discount_amount = (amount.to_f / (1 + tax.rate / 100)) if amount > 0
end

Mind you, this is the easy way, not the ideal way. A better solution is to write custom setter logic in the controller in lieu of mass-assignment; basically to manually set these attributes in the controller.