iin_checksum.rb

#
# SPECS
# 

it 'validates presence of iin' do
  form.validate(iin: '')
  expect(form.errors[:iin]).to include('must be filled')
end

it 'validates length of iin to be 12' do
  form.validate(iin: '123')
  expect(form.errors[:iin]).to include('length must be 12')
end

it 'validates checksum of iin' do
  form.validate(iin: '123456123456')
  expect(form.errors[:iin]).to include('IIN checksum is not correct')
end

it 'bypasses validation with correct iin' do
  form.validate(iin: '880909300211')
  expect(form.errors[:iin]).not_to include('IIN checksum is not correct')
end

#
# user_form.rb
#

class UserForm < Reform::Form

  property :first_name
  property :last_name
  property :iin

  validation do

    configure do
      def correct_iin?(value)
        value
          .split('')
          .first(11)
          .map
          .with_index { |n, idx| n.to_i * (idx + 1) }
          .reduce(:+) % 11 == value[11].to_i
      end
    end

    required(:first_name).filled
    required(:last_name).filled
    required(:iin) { filled? & size?(12) & correct_iin? }
  end
end

#
# ru.yml
#

ru:
  errors:
    correct_iin?: "Указанный ИНН не верный. Контрольное число не сошлось."


#
# en.yml
#

en:
  errors:
    correct_iin?: 'IIN checksum is not correct'