$ rails g devise:controllers users # => usersコントローラの作成

$ rails g model bank stock_quantity:integer user:references # =>Bankモデルの作成
$ # stock_quantityカラム：銀行残高 # user_idカラム→Userモデルから口座を呼び出す為の外部キー 
$ rails db:migrate

class User < ApplicationRecord
  has_one :bank, dependent: :destroy # ユーザーは1つの銀行口座しか開設できない
  accepts_nested_attributes_for :bank # ここがポイント

  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :validatable
end

class Bank < ApplicationRecord
  belongs_to :user
end

class ApplicationController < ActionController::Base
  before_action :configure_permitted_parameters, if: :devise_controller?

  def configure_permitted_parameters
    # 新規登録の時(sign_upアクション)に許可するパラメーター
    devise_parameter_sanitizer.permit(:sign_up, keys: [:nickname, :gender_id, :last_name, :first_name, { bank_attributes: [:stock_quantity] }])
  end
end
# { bank_attributes: [:stock_quantity] }に注目
# これでユーザー登録時にBankモデルのパラメータの許可も追加しています。

# frozen_string_literal: true

class Users::RegistrationsController < Devise::RegistrationsController
  before_action :configure_sign_up_params, only: [:create]
  #before_action :configure_account_update_params, only: [:update]

  # GET /resource/sign_up
  #def new
  #  super
  #end

  # POST /resource
  def create
    super
    @user = User.new(bank_params)
    @user.build_bank
    @user.save
  end

  # GET /resource/edit
  #def edit
  #  super
  #end

  # PUT /resource
  #def update
  #  super
  #end

  # DELETE /resource
  #def destroy
  #  super
  #end

  # GET /resource/cancel
  # Forces the session data which is usually expired after sign
  # in to be expired now. This is useful if the user wants to
  # cancel oauth signing in/up in the middle of the process,
  # removing all OAuth session data.
  #def cancel
  #  super
  #end

  # protected

  # If you have extra params to permit, append them to the sanitizer.
  def configure_sign_up_params
    devise_parameter_sanitizer.permit(:sign_up, keys: [:attribute])
  end

  # If you have extra params to permit, append them to the sanitizer.
  #def configure_account_update_params
  #  devise_parameter_sanitizer.permit(:account_update, keys: [:attribute])
  #end

  # The path used after sign up.
  def after_sign_up_path_for(resource)
    super(resource)
  end

  # The path used after sign up for inactive accounts.
  def after_inactive_sign_up_path_for(resource)
    super(resource)
  end

  private

  def bank_params
    params.permit(:sign_up, keys: [:nickname, :gender_id, :last_name, :first_name, { bank_attributes: [:stock_quantity] }])
  end
end

Rails.application.routes.draw do
  devise_for :users, controllers: {
    registrations: 'users/registrations'
  }
  root to: 'banks#index'
end

#form_with @user do |f|　配下（内部）に

#省略

# 必ずf.fields_forのヘルパーメソッドを使う
<%= f.fields_for :bank, Bank.new do |f| %>
<%= f.hidden_field :stock_quantity, value: 0 %> # hidden_fieldがミソ
<% end %>