bank.rb

BANK_RULES ||= YAML::load(ERB.new(File.read(Rails.root.join('bank_rules.yml'))).result)
BANK_RULES.default = BANK_RULES['default']

def BANK_RULES.[](key, attribute = nil)
  attributes = super(key)
  return attributes if attribute.nil?
  attributes[attribute]
end

class Bank
  include ActiveModel::Validations
  
  attr_accessor :bank_code, :agency_number, :agency_digit, :account_number, :account_digit
  validates_presence_of :bank_code, :agency_number, :agency_digit, :account_number, :account_digit
  
  validates_each BANK_RULES.keys, allow_blank: true do |record, attr, value|
    bank_rule = BANK_RULE[record.bank_code, attr]
    record.errors.add(attr, :too_long, count: bank_rule) if value.to_s.length > bank_rule
  end
end

bank_rules.yml

default: &DEFAULT
  agency_number: 10
  agency_digit: 2
  account_number: 10
  account_digit: 2
'001':
  <<: *DEFAULT
  agency_number: 4
  account_number: 8
'341':
  <<: *DEFAULT
  agency_number: 4
  account_number: 5
  account_digit: 1