Chat Using Action Cable part 3 - Chatroom and ChatroomUsers Association

I've been playing around and trying to implement a chat such as slack as shown on Chris's screencast. On Chatroom index I'm able to Show, Edit, Join, and Leave. However, when I clicked on Show even though it redirects me to the right chatroom I'm not able to send messages in real time because I haven't clicked on "joined" prior to entering the chatroom as a member. Is there a way to join the room while I'm already on the chatroom.

/views/chatrooms/show.html.erb

            *** <td><%= link_to "Join", chatroom_chatroom_users_path(@chatroom), method: :post %></td>***

            <p>
                <strong>Welcome to my chatroom:</strong>
                <%= @chatroom.name %>
            </p>

            <div data-behavior='messages' data-chatroom-id='<%= @chatroom.id %>'>
                <% @messages.each do |message| %>
                    <%= render message %>
                <% end %>
            </div>


            <%= form_for [@chatroom, Message.new] do |f| %>
                <%= f.text_area :body, rows: 1, class: "form-control", autofocus: true %>
            <% end %>

I've added *** <%= link_to "Join", chatroom_chatroom_users_path(@chatroom), method: :post %>*** on chatroom's show page to join. pic: ![http://prntscr.com/hj0egi] !

but it crashes giving me this error:![http://prntscr.com/hj0dfs]