gistfile1.rb

  def save_offer
   if request.post?
    @offer = Offer.new(params[:offer])
    @offer.post = Post.find_by_id(session[:post_id])
    @offer.save
    redirect_to :controller => 'post', :action => 'show' , :id => params[:id]
   end
  end

def add_offer #accion para agregar oferta
    session[:post_id] = params[:id] #agregamos variable de sesion para saber que post es
end