This peace of code provide validation of isbn (lenght, format, sum of number).

ActiveRecord ISBN Validation.rb

class Book < ActiveRecord::Base
  
  attr_accessor :isbn
  validate :format_of_isbn

private

  def format_of_isbn
    normalized_isbn = self.isbn.to_s.gsub(/_|-/, '')
    if normalized_isbn.length == 10
      unless normalized_isbn =~ /\d{9}(\d{1}|x)/i
        errors.add(:isbn, I18n.t('isbn_validation.wrong_format'))
        return false
      end
      numbers = normalized_isbn.split('').collect {|s| s =~ /x/i ? 10 : s.to_i }
      result = (1..10).to_a.reverse.zip(numbers).collect {|a| a.reduce(&:*)}.reduce(&:+)
      if (result % 11) == 0
        return true
      else
        errors.add(:isbn, I18n.t('isbn_validation.invalid_number'))
        return false
      end 
    elsif normalized_isbn.length == 13
      unless normalized_isbn =~ /\d{13}/i
        errors.add(:isbn, I18n.t('isbn_validation.wrong_format'))
        return false
      end
      numbers = normalized_isbn.split('').collect(&:to_i)
      result = ([1,3] * 7)[0..-2].zip(numbers).collect {|a| a.reduce(&:*)}.reduce(&:+)
      if (result % 10) == 0
        return true
      else
        errors.add(:isbn, I18n.t('isbn_validation.invalid_number'))
        return false
      end
    else
      errors.add(:isbn, I18n.t('isbn_validation.wrong_length'))
        return false
    end
  end
end