Railsのdeviseで新規登録するとき、親のモデル（belongs_toなもの）も一緒に作る方法

環境

• Rails 5.2.2
• Ruby 2.5.3

前提

• User belongs_to Organization
• Organization has_many Users
• Userを新規登録するとき、Organizationも一緒に作りたい（というか作れないとエラーで登録できない）

コード

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

  protected

  def configure_permitted_parameters
    devise_parameter_sanitizer.permit(:sign_up, keys: [organization_attributes: [:name]])
  end
end

# app/views/devise/registrations/new.html.erb
...
<% resource.organization ||= Organization.new %>
<%= simple_form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f| %>
  <%= f.error_notification %>

  <div class="form-inputs">
    <%= f.input :email,
                required: true,
                autofocus: true ,
                input_html: { autocomplete: "email" }%>
    <%= f.input :password,
                required: true,
                hint: ("#{@minimum_password_length} characters minimum" if @minimum_password_length),
                input_html: { autocomplete: "new-password" } %>
    <%= f.input :password_confirmation,
                required: true,
                input_html: { autocomplete: "new-password" } %>

    <%= f.fields_for :organization do |organization_form| %>
      <%= organization_form.input :name %>
    <% end %>
  </div>

  <div class="form-actions">
    <%= f.button :submit, t("devise.sign_up") %>
  </div>
<% end %>
...

参考

https://github.com/plataformatec/devise#strong-parameters
https://stackoverflow.com/a/7987480/7824640