Question

I get the error undefined method 'amount' for nil:NilClass if the Tax (tax_id) is not present on any products.

class Product < ActiveRecord::Base
  attr_accessible :amount, :tax_id
  belongs_to :tax

  def self.total_with_tax
    self.sum(:amount) + all.map(&:tax).map(&:amount).sum
  end
end

class Tax < ActiveRecord::Base
  attr_accessible :amount
  has_many :products
end

Is there a way I can make it so if there’s no Tax ID present when it searches all of the products, it will render it as nil and just do self.sum(:amount)?

Answer 1

Alternatively, you could scope the products to only map those that actually have a tax_id. Something like:

self.sum(:amount) + self.where("tax_id IS NOT NULL").map(&:tax).map(&:amount).sum