Flash messages in Ruby on Rails - Step-by-Step Execution

Concept Flow - Flash messages

Controller action runs

↓

Set flash message

↓

Redirect or render

↓

View reads flash message

↓

Display message to user

↓

Flash message cleared after display

Flash messages are set in the controller, passed to the next view, displayed once, then cleared automatically.

Execution Sample

Ruby on Rails

def create
  if @post.save
    flash[:notice] = "Post created!"
    redirect_to @post
  else
    flash.now[:alert] = "Error saving post"
    render :new
  end
end

This code sets a flash message on success or failure and either redirects or renders accordingly.

Execution Table

Step	Action	Flash Content Before	Flash Content After	View Rendered	Message Displayed
1	Start create action	{}	{}	None	None
2	Try to save @post	{}	{}	None	None
3	If save success	{}	{"notice": "Post created!"}	Redirect to show	Post created!
4	Flash cleared after display	{"notice": "Post created!"}	{}	Show page	None
5	If save fails	{}	{"alert": "Error saving post"}	Render new	Error saving post
6	Flash.now cleared after render	{"alert": "Error saving post"}	{}	New page	None

💡 Flash messages are cleared after being displayed once, either after redirect or render.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	After Step 6
flash	{}	{"notice": "Post created!"}	{}	{"alert": "Error saving post"}	{}

Key Moments - 2 Insights

Why does flash message disappear after one page load?

Flash messages are designed to last only for the next request. After the view reads and displays them (see step 4 and 6), Rails clears them automatically.

What is the difference between flash and flash.now?

flash sets a message for the next request (usually after redirect), while flash.now sets it for the current request (usually when rendering). See steps 3 vs 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the flash content after step 3?

A {"alert": "Error saving post"}

B {}

C {"notice": "Post created!"}

D None

💡 Hint

Check the 'Flash Content After' column at step 3.

At which step does the flash message get cleared after being displayed?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look for 'Flash cleared after display' in the 'Action' column.

If we use flash.now instead of flash in a failed save, what changes in the execution table?

A Flash message appears only in the current render, not next redirect

B Flash message persists after redirect

C Flash message never displays

D Flash message appears twice

💡 Hint

Refer to steps 5 and 6 where flash.now is used and cleared after render.

Concept Snapshot

Flash messages in Rails:
- Set in controller with flash[:key] or flash.now[:key]
- flash persists for next request (redirect)
- flash.now shows message in current render
- Messages display once then clear automatically
- Used for user notifications like success or error

Full Transcript

Flash messages in Rails help show temporary messages to users. When a controller action runs, it can set a flash message using flash[:notice] or flash[:alert]. If the action redirects, the flash message is stored and shown on the next page. After the message is displayed, Rails clears it automatically so it doesn't show again. If the action renders a view directly, flash.now can be used to show a message immediately without waiting for a redirect. This way, flash messages provide friendly feedback like 'Post created!' or 'Error saving post' that appear once and then disappear.