Action Cable for real-time updates in Ruby on Rails - Mini Project: Build & Apply

Action Cable for real-time updates

📖 Scenario: You are building a simple chat feature for a website. Users should see new messages appear instantly without refreshing the page.

🎯 Goal: Create a basic Action Cable setup in Rails that broadcasts new chat messages to all connected users in real-time.

📋 What You'll Learn

Create a channel for chat messages

Set up a subscription to the chat channel in JavaScript

Broadcast new messages from the Rails server

Display incoming messages instantly on the webpage

💡 Why This Matters

🌍 Real World

Real-time chat and notifications are common in social media, customer support, and collaboration tools.

💼 Career

Understanding Action Cable helps you build interactive Rails apps that respond instantly to user actions, a valuable skill for web developers.

1

Create the ChatChannel

Create a new channel file called app/channels/chat_channel.rb with a class ChatChannel that inherits from ApplicationCable::Channel. Inside it, define a subscribed method that streams from "chat_channel".

Ruby on Rails

# Create ChatChannel class with subscribed method
# Your code here

Hint

Use stream_from inside the subscribed method to listen to the channel named "chat_channel".

2

Subscribe to ChatChannel in JavaScript

In app/javascript/channels/chat_channel.js, create a subscription to "chat_channel" using createConsumer().subscriptions.create. Add a received function that appends the received message to an element with id "messages".

Ruby on Rails

class ChatChannel < ApplicationCable::Channel
  def subscribed
    stream_from "chat_channel"
  end
end

// Subscribe to chat_channel and append messages
// Your code here

Hint

Use subscriptions.create("ChatChannel", { received(data) { ... } }) to handle incoming messages.

3

Broadcast messages from Rails

In your MessagesController, after saving a new message, broadcast it to "chat_channel" using ActionCable.server.broadcast with a hash containing the key :message and the message text.

Ruby on Rails

class ChatChannel < ApplicationCable::Channel
  def subscribed
    stream_from "chat_channel"
  end
end

import { createConsumer } from "@rails/actioncable"

const consumer = createConsumer()

consumer.subscriptions.create("ChatChannel", {
  received(data) {
    const messages = document.getElementById("messages")
    const messageElement = document.createElement("p")
    messageElement.textContent = data.message
    messages.appendChild(messageElement)
  }
})

class MessagesController < ApplicationController
  def create
    @message = Message.new(message_params)
    if @message.save
      # Your code here
    end
  end

  private

  def message_params
    params.require(:message).permit(:content)
  end
end

Hint

Use ActionCable.server.broadcast("chat_channel", message: @message.content) to send the new message to all subscribers.

4

Add messages container in HTML

In your chat view file (e.g., app/views/messages/index.html.erb), add a <div> with id "messages" where incoming messages will appear.

Ruby on Rails

class ChatChannel 

Hint

Add a container with id="messages" so JavaScript can add new messages inside it.

Practice

1. What is the main purpose of Action Cable in a Rails application?

easy

A. To add real-time features allowing live updates without page reloads

B. To manage database migrations automatically

C. To handle background jobs and scheduling

D. To style the application with CSS frameworks

Solution

1. Step 1: Understand Action Cable's role

   Action Cable integrates WebSockets into Rails to enable live, real-time communication.

2. Step 2: Compare options with Action Cable's purpose

   Only To add real-time features allowing live updates without page reloads describes real-time updates without page reloads, which is what Action Cable does.

3. Final Answer:

   To add real-time features allowing live updates without page reloads -> Option A

4. Quick Check:

   Real-time updates = To add real-time features allowing live updates without page reloads [OK]

Hint: Remember: Action Cable = live updates without refreshing [OK]

Common Mistakes:

• Confusing Action Cable with background job tools
• Thinking it manages database tasks
• Assuming it styles the app

2. Which of the following is the correct way to create a channel subscription in a Rails JavaScript client using Action Cable?

easy

A. App.cable.subscriptions.create('ChatChannel')

B. App.cable.subscribe('ChatChannel')

C. App.cable.connect('ChatChannel')

D. App.cable.open('ChatChannel')

Solution

1. Step 1: Recall Action Cable subscription syntax

   The correct method to subscribe to a channel is using subscriptions.create.

2. Step 2: Match options with correct method

   Only App.cable.subscriptions.create('ChatChannel') uses subscriptions.create('ChatChannel'), which is the correct syntax.

3. Final Answer:

   App.cable.subscriptions.create('ChatChannel') -> Option A

4. Quick Check:

   Subscription method = subscriptions.create [OK]

Hint: Use subscriptions.create to subscribe to channels [OK]

Common Mistakes:

• Using subscribe instead of subscriptions.create
• Using connect or open which are invalid methods
• Missing quotes around channel name

3. Given this channel code in Rails:

class ChatChannel < ApplicationCable::Channel
  def subscribed
    stream_from 'chat_room'
  end
end

And this JavaScript subscription:

App.cable.subscriptions.create('ChatChannel', {
  received(data) {
    console.log(data.message)
  }
})

What will happen when the server broadcasts { message: 'Hello!' } to 'chat_room'?

medium

A. Nothing will happen because the channel name is incorrect

B. The message 'Hello!' will be logged in the browser console

C. The message will be displayed as an alert popup

D. The server will throw an error due to missing perform method

Solution

1. Step 1: Understand the channel subscription and streaming

   The channel streams from 'chat_room', and the client subscribes to 'ChatChannel' with a received callback logging data.message.

2. Step 2: Analyze the broadcast effect

   When the server broadcasts { message: 'Hello!' } to 'chat_room', the client receives it and logs 'Hello!' to the console.

3. Final Answer:

   The message 'Hello!' will be logged in the browser console -> Option B

4. Quick Check:

   Broadcast to stream triggers received callback = The message 'Hello!' will be logged in the browser console [OK]

Hint: Broadcast to stream triggers received callback logging message [OK]

Common Mistakes:

• Confusing channel name with stream name
• Expecting alert instead of console log
• Thinking server errors without perform method

4. Identify the error in this Action Cable channel code:

class NotificationsChannel < ApplicationCable::Channel
  def subscribed
    stream_from notifications
  end
end

medium

A. The channel class should inherit from ApplicationController

B. The subscribed method is missing a call to reject unauthorized users

C. The stream_from argument should be a string, not a variable

D. The stream_from method is deprecated and should not be used

Solution

1. Step 1: Check the stream_from argument type

   The argument to stream_from must be a string representing the stream name, e.g., 'notifications'.

2. Step 2: Identify the error in the code

   The code uses notifications without quotes, treating it as a variable, which causes an error.

3. Final Answer:

   The stream_from argument should be a string, not a variable -> Option C

4. Quick Check:

   stream_from needs string argument [OK]

Hint: Always quote stream names in stream_from method [OK]

Common Mistakes:

• Forgetting quotes around stream name
• Inheriting from wrong class
• Assuming stream_from is deprecated

5. You want to broadcast a message to all users subscribed to 'notifications' channel whenever a new comment is created. Which code snippet correctly achieves this in Rails?

hard

A. ActionCable.broadcast('notifications', @comment.body)

B. NotificationsChannel.broadcast_to('notifications', { comment: @comment.body })

C. broadcast_to('notifications', { comment: @comment.body })

D. ActionCable.server.broadcast('notifications', { comment: @comment.body })

Solution

1. Step 1: Identify the correct broadcast method

   To send a message to all subscribers of a stream, use ActionCable.server.broadcast(stream, data).

2. Step 2: Analyze options for correct usage

   ActionCable.server.broadcast('notifications', { comment: @comment.body }) uses ActionCable.server.broadcast with stream name and data hash, which is correct.

3. Final Answer:

   ActionCable.server.broadcast('notifications', { comment: @comment.body }) -> Option D

4. Quick Check:

   Broadcast to stream uses ActionCable.server.broadcast [OK]

Hint: Use ActionCable.server.broadcast to send to all subscribers [OK]

Common Mistakes:

• Using broadcast_to without a model instance
• Calling broadcast on ActionCable directly
• Passing data without wrapping in a hash