card_assignment.rb

class Card
  attr_accessor :new_employee_id

  def reassign(employee_id)
    old_card = self.card_assignments.all(:conditions=>["active_since < #{DateTime.now} and #{DateTime.now} < active_until"]).first

    if !old_card.nil?
      old_card.active_until = DateTime.now
      old_card.save
    end

    active_card = CardAssignment.new(:card_id => self.id, :employee_id => employee_id)
    active_card.active_since = DateTime.now
    active_card.active_until = DateTime.now + 1.year
    active_card.first_time = true
    active_card.save
  end
end

cards.rb

def update
  @card = Card.find(params[:id])

  if @card.update(params[:card])
    @card.reassign(params[:card][:employee_id])

  else
  end
end