transition_validator.rb

class TransitionValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, value)
    return unless record.changes[attribute]
    transitions = options
    start = record.send(:"#{attribute}_was")
    destination = value
    message = options[:message] || "can not be transitioned from #{start.inspect} to #{destination.inspect}"

    allowed_transitions = transitions[start]
    unless allowed_transitions
      raise Error, record.errors.full_message(attribute, message)
    end
    unless Array(allowed_transitions).map(&:to_sym).include?(destination.to_sym)
      raise Error, record.errors.full_message(attribute, message)
    end
  end

  class Error < StandardError
  end

end