how to shorten this ?

 def integrity
    if time_credit?
      errors.add(:base, I18n.t(:establishment_cost_empty, :scope => [:models, :destinations, :integrity])) unless establishment_cost?
    elsif establishment_cost?
      errors.add(:base, I18n.t(:time_credit_empty, :scope => [:models, :destinations, :integrity])) unless time_credit?
    end
  end

unless (time_credit? && cost_empty?) || !(time_credit? || cost_empty?)
field_in_error = time_credit.empt? ? :time_credit : :cost_empty
errors.add(field_in_error, I18n.t(field_in_error, :scope => ...))
end