generate migrate add_user_id_to_group
rake db:migrate add_user_id_to_group
generate user and group database relationship
groups/create, edit, update, destroy 加入 User_id 參數
只有 current_user 跟 作者相同才能在前台顯示 edit / destroy 按鈕
generate migrate add_user_id_to_post
rake db:migrate add_user_id_to_post