Redirect and render in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does redirect_to do in Rails?

redirect_to sends the browser to a different URL, causing a new request. It does not render a view directly.

beginner

What is the difference between render and redirect_to in Rails?

render shows a view template without changing the URL or making a new request.
redirect_to sends a new request to a different URL, changing the browser's address.

intermediate

Can you use render and redirect_to together in the same controller action?

No, you should use only one. render displays a view, while redirect_to sends a new request. Using both causes errors.

intermediate

What happens if you call redirect_to after render in a Rails action?

Rails raises a DoubleRenderError because you can only send one response per request.

beginner

How do you redirect to a named route in Rails?

Use redirect_to with the route helper, like redirect_to posts_path to go to the posts index page.

What does render :edit do in a Rails controller?

A Redirects the browser to the edit URL

B Displays the edit view without changing the URL

C Reloads the current page

D Sends a JSON response

render :edit shows the edit template without a new request or URL change.

Which method causes the browser to make a new request to a different URL?

A redirect_to

B render

C flash

D params

redirect_to sends a new HTTP request to the specified URL.

What error occurs if you call both render and redirect_to in one action?

A NoMethodError

B ArgumentError

C SyntaxError

D DoubleRenderError

Rails raises DoubleRenderError because only one response is allowed per request.

How do you redirect to the root path in Rails?

A redirect_to root_path

B render root_path

C redirect root_path

D render_to root_path

Use redirect_to root_path to send the browser to the home page.

Which of these is true about render?

A It changes the browser URL

B It sends a new HTTP request

C It displays a view template without a new request

D It redirects to another controller

render shows a view template in the current request without changing the URL.

Explain in your own words the difference between redirect_to and render in Rails controllers.

Think about what the browser does after each method is called.

Describe what happens if you try to use both render and redirect_to in the same Rails action.

Rails does not allow sending two responses for one request.

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