ABA Routing Number Validator for Rails

routing_number_validator.rb

# frozen_string_literal: true

class RoutingNumberValidator < ActiveModel::EachValidator

  WEIGHTS = ([3, 7, 1] * 3).freeze
  DIVISOR = 10

  def validate_each(record, attribute, value)
    return if value.blank?

    @record    = record
    @attribute = attribute
    @value     = value

    # TODO: Canada routing number validation missing
    # See https://github.com/activemerchant/active_merchant/blob/master/lib/active_merchant/billing/check.rb

    case options[:format]
    when :aba
      validate_numericality
      validate_length(9)
      validate_checksum
    else
      raise ArgumentError, "#{self.class} does not support format '#{options[:format]}'"
    end
  end

private

  def validate_numericality
    ActiveModel::Validations::NumericalityValidator.new(
      attributes:   @attribute,
      only_integer: true
    ).validate(@record)
  end

  def validate_length(length)
    ActiveModel::Validations::LengthValidator.new(
      attributes: @attribute,
      is:         length
    ).validate(@record)
  end

  def validate_checksum
    return if checksum.zero?

    @record.errors.add(@attribute, :invalid_routing_number, **options)
  end

  def checksum
    return 1 if @record.errors[@attribute].any?

    @value.chars.map(&:to_i).zip(WEIGHTS).sum { |digit, weight| digit * weight } % DIVISOR
  end

end

Credit to @JessCodes for the collaboration

Usage

validates :routing_number, routing_number: { format: :aba }, presence: true

• Attribute must be type String.
• presence option not required.
• Define in config/locales/errors.en.yml or similar:

  en:
    errors:
      messages:
        invalid_routing_number: "My custom error message here"