RailsHow-ToBeginner · 3 min read

How to Use Flash Messages in Rails: Simple Guide

In Rails, use the flash hash to set temporary messages that last for one request cycle. Assign messages like flash[:notice] = 'Success!' in controllers, then display them in views with flash[:notice] or flash[:alert].

📐

Syntax

The flash is a special hash in Rails used to store messages that persist for one request. You set messages in the controller and read them in the view.

flash[:notice] = 'Message': Sets a success or info message.
flash[:alert] = 'Message': Sets a warning or error message.
In the view, use flash[:notice] or flash[:alert] to show the message.

ruby

class PostsController < ApplicationController
  def create
    @post = Post.new(post_params)
    if @post.save
      flash[:notice] = 'Post was successfully created.'
      redirect_to @post
    else
      flash[:alert] = 'Error creating post.'
      render :new
    end
  end

  private

  def post_params
    params.require(:post).permit(:title, :content)
  end
end

💻

Example

This example shows how to set a flash message in the controller after creating a post, then display it in the view layout.

ruby,erb

# app/controllers/posts_controller.rb
class PostsController < ApplicationController
  def create
    @post = Post.new(post_params)
    if @post.save
      flash[:notice] = 'Post created successfully!'
      redirect_to @post
    else
      flash[:alert] = 'Failed to create post.'
      render :new
    end
  end

  private

  def post_params
    params.require(:post).permit(:title, :content)
  end
end

# app/views/layouts/application.html.erb
<!DOCTYPE html>
<html lang="en">
<head>
  <title>My Blog</title>
</head>
<body>
  <% if flash[:notice] %>
    <p style="color: green;"><%= flash[:notice] %></p>
  <% end %>
  <% if flash[:alert] %>
    <p style="color: red;"><%= flash[:alert] %></p>
  <% end %>

  <%= yield %>
</body>
</html>

Output

When a post is created successfully, the page shows a green message: "Post created successfully!". If creation fails, a red message: "Failed to create post." appears.

⚠️

Common Pitfalls

Setting flash messages but forgetting to redirect causes the message to persist longer than expected.
Using render instead of redirect_to keeps the same request, so flash messages may not clear as expected.
Not checking if flash[:notice] or flash[:alert] exists before displaying can cause empty message areas.

ruby

class PostsController < ApplicationController
  def create
    @post = Post.new(post_params)
    if @post.save
      flash[:notice] = 'Post created!'
      redirect_to @post # Correct: flash will show once
    else
      flash[:alert] = 'Error!'
      render :new # Flash persists only for this render
    end
  end

  private

  def post_params
    params.require(:post).permit(:title, :content)
  end
end

📊

Quick Reference

Flash Key	Purpose	Typical Use
notice	Informational or success messages	flash[:notice] = 'Saved successfully'
alert	Warnings or error messages	flash[:alert] = 'Something went wrong'
now	Shows message only for current request (no redirect)	flash.now[:alert] = 'Fix errors'

✅

Key Takeaways

Use flash[:notice] and flash[:alert] in controllers to set messages for the next request.

Display flash messages in views by checking if flash[:notice] or flash[:alert] exists.

Use redirect_to after setting flash to show messages on the next page load.

Use flash.now when rendering the same page to show messages immediately without redirect.

Always check for flash presence in views to avoid empty message areas.