find_by アソシエーション

Question

find_byメソッドでアソシエーションを使いましょうと言われたのですが、調べてもどうすればいいかわからないのでご教授願います

class CardController < ApplicationController

  require "payjp"

  def new
    card = Card.where(user_id: current_user.id)
    redirect_to action: "show" if card.exists?
  end
  
  def pay #payjpとCardのデータベース作成
    Payjp.api_key = ENV["PAYJP_PRIVATE_KEY"]
    if params['payjp-token'].blank?
      redirect_to action: "new"
    else
      customer = Payjp::Customer.create(
      description: '登録テスト', 
      email: current_user.email, 
      card: params['payjp-token'],
      metadata: {user_id: current_user.id}
      ) 
      @card = Card.new(user_id: current_user.id, customer_id: customer.id, card_id: customer.default_card)
      if @card.save
        redirect_to action: "show"
      else
        redirect_to action: "pay"
      end
    end
  end

  def delete #PayjpとCardデータベースを削除
    card = Card.find_by(user_id: current_user.id)
    if card.blank?
    else
      Payjp.api_key = ENV["PAYJP_PRIVATE_KEY"]
      customer = Payjp::Customer.retrieve(card.customer_id)
      customer.delete
      card.delete
    end
      redirect_to  controller: :green , action: :pay
  end

  def show #Cardのデータpayjpに送り情報を取り出し
    card = Card.find_by(user_id: current_user.id)
    if card.blank?
      redirect_to action: "new" 
    else
      Payjp.api_key = ENV["PAYJP_PRIVATE_KEY"]
      customer = Payjp::Customer.retrieve(card.customer_id)
      @default_card_information = customer.cards.retrieve(card.card_id)
    end
  end
end

Answer 1

Card.find_by(user_id: current_user.id)

を

card = current_user.card
# もしくは
card = current_user.cards.first

に書き換えられるかもしれない という指摘だと思います。