Including validation method in another method - ruby-on-rails

Question

I have a validation method that has to verify values assigned in another method, how can i get it to recognise those values before validation? the pay_must_be_same_to_amount method needs some values from the create_items_from_readings method
class Invoice < ActiveRecord::Base
attr_accessible :approved_by, :due_date, :invoice_date, :reading_ids, :terms, :customer_id, :customer, :status, :reference_no, :payment_method, :amount, :payment_date
has_many :invoice_items, :dependent => :destroy
belongs_to :customer, :inverse_of => :invoices
validate :pay_must_be_same_to_amount
def create_item_from_readings
item = invoice_items.new
item.rate = customer.unit_cost
readings_in_this_period = customer.unbilled_readings.where('date_of_reading <= ?', invoice_date).order(:date_of_reading)
return nil if readings_in_this_period.empty?
self.reading_ids = readings_in_this_period.collect(&:id).join(',')
total_units = 0
readings_in_this_period.each do |reading|
total_units = total_units + reading.units_used1 + reading.units_used2 + reading.units_used3
end
item.amount = total_units * customer.unit_cost * customer.ct_ratio
item.tax_amount = (item.amount * Settings.vat) if customer.pays_vat
invoice_from_reading = readings_in_this_period.first.previous_reading
invoice_from_reading ||= readings_in_this_period.first
invoice_to_reading = readings_in_this_period.last
#Select Item description based on Phase type
if customer.phase_type == 'Single Phase'
item.description = "Electricity used from #{invoice_from_reading.date_of_reading.strftime('%d/%m/%Y')} with readings #{invoice_from_reading.reading1} to #{invoice_to_reading.date_of_reading.strftime('%d/%m/%Y')} with reading #{invoice_to_reading.reading1} - #{total_units.to_i} total units"
else
item.description = "Electricity used from #{invoice_from_reading.date_of_reading.strftime('%d/%m/%Y')} with readings, R1: #{invoice_from_reading.reading1}, R2: #{invoice_from_reading.reading2}, R3: #{invoice_from_reading.reading3} to #{invoice_to_reading.date_of_reading.strftime('%d/%m/%Y')} with readings, R1: #{invoice_to_reading.reading1}, R2:#{invoice_to_reading.reading2}, R3: # {invoice_to_reading.reading3}- #{total_units.to_i} total units"
end
end
end
and the validation method is below, it needs to compare the item.amount above to the amount in the class Invoice
def pay_must_be_same_to_amount
if item.amount < self.amount && item.amount != self.amount
self.errors.add :amount, 'The payment amount should be equal to amount on invoice'
end
end
end

Answer 1

A few comments: create_item_from_readings is way too complicated. I can't tell what it's supposed to do, but if you run it, I believe it will return a string (one of the two from the last if statement).
If all you need to do is compare item.amount to the invoice amount attribute, that's simple. You can use your validation method almost as you've written it, plus a few other methods as needed.
def item_amount
total_units * customer.unit_cost * customer.ct_ratio
end
def total_units
readings_in_this_period.inject(0) {|total,r| total + r.units_used1 + r.units_used2 + r.units_used3 }
end
def pay_must_be_same_to_amount
if item_amount != amount
errors.add :amount, 'The payment amount should be equal to amount on invoice'
end
end
The code for both of those supplementary methods is simply modified code from your longer method.
A good rule of practice is that if a method is longer than one line, and you can't tell what it's for by glancing at it, it's too long (this isn't always true, but it's worth considering for complicated methods).

Answer 2

The solution to the question is
def pay_must_be_same_to_amount
sum = 0
self.invoice_items.each do |invoice_item|
sum = sum + invoice_item.amount
end
if sum != self.amount
self.errors.add :amount, 'The payment amount should be equal to amount on invoice'
end
end