Rails rendering why have to call .to_json

I work on a Rails API, and I want to render some binary in json. To do that I convert my binary in hex to render it.

So I have

#<PlayCard id: 3, card_id: 12, atk: 10, hp: 9, deck_id: nil, game_id: nil, uid: ".dk\x8A", created_at: "2018-06-06 15:17:25", updated_at: "2018-06-06 15:17:25", user_id: 27>
(byebug) play_card.to_json
{"id"=>3, "card_id"=>12, "atk"=>10, "hp"=>9, "deck_id"=>nil, "game_id"=>nil, "uid"=>"2e646b8a", "created_at"=>Wed, 06 Jun 2018 17:17:25 CEST +02:00, "updated_at"=>Wed, 06 Jun 2018 17:17:25 CEST +02:00, "user_id"=>27}

My question is about the rendering of my object. With my method show I Have no problem but with my method create I have to call my_object.to_json did you have an idea ? With out the .to_json I have a ActionDispatch::TestResponse object.

def show
  record = PlayCard.find_by(id: params[:id])
  if record.present?
    render json: record.attributes.except('uid'), status: :ok
  else
    render json: {}, status: :no_content
  end
end

def create
  play_card = PlayCardsService.create(play_card_params)
  if play_card.valid?
    render json: play_card.to_json, status: :created
  else
    render json: { status: 'KO', errors: play_card.errors.full_messages }, status: :unprocessable_entity
  end
end

class PlayCardsService

  class << self

    def create(play_card_params)
      PlayCard.create(play_card_params)
    end

  end

end

def to_json(options = {})
  bin = bin_to_hex(self.uid)
  self.uid = nil
  json = self.as_json
  json['uid'] = bin
  json
end

def bin_to_hex(s)
  s.each_byte.map { |b| b.to_s(16).rjust(2,'0') }.join
end

Thanks for your help

Have a nice day,