How do I use rails validates for length to check for one value or another, not a range?

Question

I am trying to write a model for books in my rails app and I want to validate the isbn attribute but there are two possible lengths for an ISBN: 10 and 13. How do I use validates to make sure that the given isbn is either 10 OR 13 numbers long?

I thought about using a range:

validates :isbn, length: { minimum: 10, maximum: 13 }

but if it is somehow 11 or 12 numbers it { should_not be_valid }.

Is there a way to do this?

Answer 1

You can use a custom validator for that purpose:

class Book < ActiveRecord::Base
  attr_accessible :isbn

  validate :check_length

  def check_length
    unless isbn.size == 10 or isbn.size == 13
      errors.add(:isbn, "length must be 10 or 13") 
    end
  end
end