check_integrity.rb

  def integrity
    dependent_attributes = %w(time_credit establishment_cost)
    nullity_proc = lambda{|attr| send(attr.to_sym).nil? || send(attr.to_sym).zero?}
    dependent_attributes.each_index do |i|
      if nullity_proc.call(dependent_attributes[i]) && !nullity_proc.call(dependent_attributes[i-1])
        errors.add(dependent_attributes[i].to_sym, I18n.t("#{dependent_attributes[i]}_empty", :scope => [:models, :destinations, :integrity]))
      end
    end
  end