Redirect and render in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when both redirect_to and render are called in a Rails controller action?

Consider this Rails controller action:

def example
  redirect_to root_path
  render :show
end

What will be the result when this action is called?

Ruby on Rails

def example
  redirect_to root_path
  render :show
end

A The action redirects to root_path and then renders the show template, so both happen.

B Rails ignores the render call and only performs the redirect_to.

C Rails ignores the redirect_to and only renders the show template.

D Rails raises a DoubleRenderError because you cannot call both redirect_to and render in the same action.

💡 Hint

Think about what Rails allows in a single controller action regarding response.

Rails does not allow both redirect_to and render to be called in the same action because it causes confusion about what response to send. It raises a DoubleRenderError.

❓ state_output

intermediate

What is the HTTP status code when using redirect_to in Rails without specifying status?

In a Rails controller, if you call redirect_to some_path without specifying a status, what HTTP status code is sent to the browser?

Ruby on Rails

redirect_to some_path

A 200 OK

B 301 Moved Permanently

C 302 Found (Temporary Redirect)

D 404 Not Found

💡 Hint

Think about the default redirect status Rails uses for redirect_to.

By default, redirect_to sends a 302 Found status, which means a temporary redirect.

📝 Syntax

advanced

Which option correctly renders a JSON response and then redirects in a Rails controller?

Given the following controller code, which option will NOT cause an error?

def example
  # your code here
end

Ruby on Rails

def example
  # your code here
end

A redirect_to root_path

B

render json: { message: 'Hello' }
redirect_to root_path

C

redirect_to root_path
render json: { message: 'Hello' }

D render json: { message: 'Hello' }

💡 Hint

Remember Rails only allows one response per action.

Calling both render and redirect_to in the same action causes a DoubleRenderError. Only one response is allowed. Options A and B call both, causing errors. Option A only renders JSON, which is valid. Option A only redirects, which is also valid. But the question asks which option will NOT cause an error when rendering JSON and then redirecting. Since you cannot do both, only option A is valid as it only renders JSON. Option A only redirects, which is also valid but does not render JSON.

🔧 Debug

advanced

Why does this Rails controller action raise a DoubleRenderError?

Look at this controller action:

def show
  if params[:format] == 'json'
    render json: { message: 'Hello' }
  end
  redirect_to root_path
end

Why does it raise a DoubleRenderError?

Ruby on Rails

def show
  if params[:format] == 'json'
    render json: { message: 'Hello' }
  end
  redirect_to root_path
end

A Because <code>redirect_to</code> is called after <code>render</code> without stopping execution.

B Because <code>render</code> is missing a status code.

C Because <code>redirect_to</code> cannot be used with JSON responses.

D Because the <code>if</code> condition is always true.

💡 Hint

Think about what happens after render is called.

After calling render, the action continues and calls redirect_to. Rails does not allow sending two responses, so it raises a DoubleRenderError. To fix, you must return or use elsif to avoid calling both.

🧠 Conceptual

expert

How does Rails decide what to do when both render and redirect_to are called conditionally in the same action?

Consider this controller action:

def index
  if params[:redirect]
    redirect_to root_path
  else
    render :index
  end
end

What will Rails do when params[:redirect] is true and when it is false?

Ruby on Rails

def index
  if params[:redirect]
    redirect_to root_path
  else
    render :index
  end
end

A Rails always renders the index template regardless of the condition.

B When true, Rails redirects; when false, Rails renders the index template. No errors occur.

C Rails raises a DoubleRenderError because both render and redirect_to exist in the action.

D Rails ignores the redirect_to and always renders the index template.

💡 Hint

Think about conditional flow and response in Rails actions.

Rails allows either render or redirect_to to be called once per action. Here, only one is called depending on the condition, so no error occurs.

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