Redirect and render in Ruby on Rails - Performance & Optimization

Performance: Redirect and render

MEDIUM IMPACT

This concept affects page load speed and user experience by controlling how the browser navigates or updates content after a server response.

Updating page content after form submission

Ruby on Rails

def create
  if @item.save
    render :show
  else
    render :new
  end
end

Render updates the current page content without a new HTTP request, speeding up content display.

📈 Performance GainSingle HTTP response, no extra page reload, reduces LCP and improves perceived speed.

Updating page content after form submission

Ruby on Rails

def create
  if @item.save
    redirect_to items_path
  else
    render :new
  end
end

Redirect causes a full new HTTP request and page reload, increasing load time and delaying content display.

📉 Performance CostTriggers 2 HTTP requests and 2 page loads, increasing LCP and blocking rendering longer.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Redirect	Full DOM rebuild	Multiple reflows due to full reload	High paint cost	[X] Bad
Render	Partial DOM update	Minimal reflows	Lower paint cost	[OK] Good

Rendering Pipeline

Redirect sends a 3xx HTTP status causing the browser to request a new page, triggering full reload and re-render. Render sends HTML directly in the response, updating the current page content without reload.

→Network Request

→HTML Parsing

→Style Calculation

→Layout

→Paint

⚠️ BottleneckNetwork Request and HTML Parsing due to full page reload in redirect

Core Web Vital Affected

LCP

This concept affects page load speed and user experience by controlling how the browser navigates or updates content after a server response.

Optimization Tips

1Use render to update page content without a full reload for better speed.

2Use redirect only when you need to navigate to a different URL.

3Avoid unnecessary redirects to reduce network requests and improve LCP.

Performance Quiz - 3 Questions

Which method causes the browser to make a new HTTP request and reload the page?

A render

B both redirect and render

C redirect

D neither redirect nor render

DevTools: Network

How to check: Open DevTools, go to Network tab, submit form and observe if multiple requests occur or just one response with HTML.

What to look for: Multiple HTTP requests indicate redirect; single HTML response indicates render, which is better for performance.

Practice

1. What does redirect_to do in a Rails controller?

easy

A. Stops the server from processing the request

B. Displays a view template without changing the URL

C. Sends the user to a different URL and starts a new request

D. Reloads the current page without any changes

Solution

1. Step 1: Understand the purpose of redirect_to

   redirect_to tells the browser to go to a new URL, causing a new HTTP request.

2. Step 2: Compare with render

   render shows a view without changing the URL or making a new request, unlike redirect_to.

3. Final Answer:

   Sends the user to a different URL and starts a new request -> Option C

4. Quick Check:

   redirect_to = new request and URL change [OK]

Hint: Redirect changes URL and request; render does not [OK]

Common Mistakes:

• Confusing render with redirect_to
• Thinking redirect_to does not change URL
• Assuming redirect_to renders a view template

2. Which of the following is the correct syntax to render the edit template in a Rails controller?

easy

A. redirect_to :edit

B. render :edit

C. render_to :edit

D. redirect :edit

Solution

1. Step 1: Recall the syntax for rendering templates

   In Rails, to show a view template without redirecting, use render :template_name.

2. Step 2: Identify the correct option

   Only render :edit is valid syntax to render the edit view.

3. Final Answer:

   render :edit -> Option B

4. Quick Check:

   render :template_name syntax = render :edit [OK]

Hint: Use render with colon and template name to show views [OK]

Common Mistakes:

• Using redirect_to instead of render to show a view
• Using incorrect method names like render_to
• Missing colon before template name

3. Given this Rails controller code:

def create
  @post = Post.new(post_params)
  if @post.save
    redirect_to @post
  else
    render :new
  end
end

What happens when @post.save returns false?

medium

A. The browser reloads the current page without changes

B. The user is redirected to the post's show page

C. The server raises an exception

D. The new post form is shown again with errors

Solution

1. Step 1: Analyze the if-else condition

   If @post.save is true, it redirects to the post's show page. If false, it renders the new template.

2. Step 2: Understand render behavior on failure

   Rendering :new shows the form again, usually with error messages, without changing the URL.

3. Final Answer:

   The new post form is shown again with errors -> Option D

4. Quick Check:

   save false triggers render :new = The new post form is shown again with errors [OK]

Hint: If save fails, render form again; if success, redirect [OK]

Common Mistakes:

• Thinking redirect happens on save failure
• Assuming an error is raised automatically
• Confusing render with redirect behavior

4. Identify the error in this Rails controller code:

def update
  @user = User.find(params[:id])
  if @user.update(user_params)
    render redirect_to @user
  else
    render :edit
  end
end

medium

A. Using render redirect_to @user is invalid syntax

B. Missing end for the method

C. Should use redirect_to :edit instead of render :edit

D. No error, code works fine

Solution

1. Step 1: Check the line with render redirect_to @user

   This line tries to combine render and redirect_to, which is invalid syntax in Rails.

2. Step 2: Correct usage of redirect and render

   Use either redirect_to @user alone to redirect, or render :template to render a view. They cannot be combined.

3. Final Answer:

   Using render redirect_to @user is invalid syntax -> Option A

4. Quick Check:

   render and redirect_to cannot be combined [OK]

Hint: Use either render or redirect_to, never both together [OK]

Common Mistakes:

• Combining render and redirect_to in one line
• Confusing render :edit with redirect_to :edit
• Forgetting to end method blocks

5. You want to show a form with errors after a failed update, but keep the URL unchanged. Which is the best approach in Rails?

hard

A. Use render :edit to show the form with errors on the same URL

B. Use redirect_to @user to show the user's profile page

C. Use redirect_to edit_user_path(@user) to reload the form

D. Use render redirect_to :edit to combine both

Solution

1. Step 1: Understand URL behavior for forms with errors

   To keep the URL unchanged and show errors, you must render the form template again without redirecting.

2. Step 2: Choose the correct method

   render :edit displays the edit form with errors on the same URL, while redirect_to changes the URL and starts a new request, losing error info.

3. Final Answer:

   Use render :edit to show the form with errors on the same URL -> Option A

4. Quick Check:

   Render keeps URL, redirect changes it [OK]

Hint: Render to keep URL and show errors; redirect loses error data [OK]

Common Mistakes:

• Using redirect_to which loses error messages
• Trying to combine render and redirect_to
• Assuming redirect_to reloads form with errors