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

• syntax on line 2: params is a hash not a method... so access it like you are on line 3
• line 2: need to check if state is present so it doesn't break when it's not there.
• line 2: state will be a string so == used won't work
• to query with state, .all() won't take a param. use Coupon.where(:state => params[:state])

def index
if params.has_key?(:state) && params[:state] == "used"
@coupons = Coupon.where(:state => params[:state])
else
@coupons = Coupon.all
end

respond_to do |format|
  format.html # index.html.erb
  format.json { render json: @coupons }
end

end

  def index 
    if params[:state] == "used"
      @coupons = Coupon.where(:state => params[:state])
    else
      @coupons = Coupon.all
    end

    respond_to do |format|
      format.html # index.html.erb
      format.json { render json: @coupons }
    end
  end

You don't actually need the has_key? check in this case

lgtm