index

coupons_controller.rb

  def index 
    if params(:state) == used
      @coupons = Coupon.all(params[:state])
    else
      @coupons = Coupon.all
    end
    
    respond_to do |format|
      format.html # index.html.erb
      format.json { render json: @coupons }
    end
  end

lgtm