Route parameters in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Rails route helper?

Given the route get '/books/:id', to: 'books#show' and the controller action def show; render plain: params[:id]; end, what will be the output when visiting /books/42?

A The page will display the text '42'.

B The page will display the text ':id'.

C The page will show a routing error.

D The page will display the text 'books#show'.

💡 Hint

Think about how route parameters are passed to the controller.

The route captures the segment after '/books/' as the :id parameter. The controller action renders this value, so visiting '/books/42' shows '42'.

📝 Syntax

intermediate

Which route definition correctly captures a username parameter?

You want to define a route that matches URLs like /profile/johndoe and passes 'johndoe' as a parameter named username. Which route definition is correct?

A get '/profile/*username', to: 'users#show'

B get '/profile/username', to: 'users#show'

C get '/profile/:username', to: 'users#show'

D get '/profile/:id', to: 'users#show'

💡 Hint

Route parameters use a colon before the name.

Using ':username' captures that part of the URL as params[:username]. Option C treats 'username' as a fixed string, A captures a wildcard but not as a named parameter, and D uses a different parameter name.

🔧 Debug

advanced

Why does this route not capture the parameter?

Given the route get '/items/:item_id/edit', to: 'items#edit', why does visiting /items/edit cause a routing error?

A Because the controller action 'edit' is missing.

B Because the route should be defined as 'get '/items/edit/:item_id''.

C Because the route should use a query parameter instead of a route parameter.

D Because the route expects a segment between '/items/' and '/edit' to capture as :item_id, but '/items/edit' has none.

💡 Hint

Look at the URL structure and the route pattern carefully.

The route expects a URL like '/items/123/edit' where '123' is the :item_id. Visiting '/items/edit' misses that segment, so Rails cannot match the route.

❓ state_output

advanced

What is the value of params in this nested route?

Given the nested route resources :authors do resources :books end and visiting /authors/5/books/10, what will be the values of params[:author_id] and params[:id] in the BooksController?

A params[:author_id] is '5' and params[:id] is '10'.

B params[:author_id] is '10' and params[:id] is '5'.

C params[:author_id] and params[:id] are both '5'.

D params[:author_id] and params[:id] are both '10'.

💡 Hint

Nested resources pass the parent resource id with _id suffix.

In nested routes, the parent resource id is passed as params[:author_id], and the nested resource id is params[:id]. So visiting '/authors/5/books/10' sets author_id to '5' and id to '10'.

🧠 Conceptual

expert

What error occurs if a required route parameter is missing?

If a Rails route is defined as get '/orders/:order_id/items/:id', to: 'items#show' and a request is made to /orders/15/items (missing the :id), what error will Rails raise?

A ActionController::UrlGenerationError

B RoutingError

C NoMethodError

D ActiveRecord::RecordNotFound

💡 Hint

Think about how Rails matches URLs to routes.

Rails cannot match the URL '/orders/15/items' to the route requiring both :order_id and :id, so it raises a RoutingError indicating no route matches.

Practice

1. What does a route parameter in Rails do?
get '/users/:id', to: 'users#show'

easy

A. Captures part of the URL as a variable to use in the controller

B. Defines a fixed URL path without variables

C. Specifies the HTTP method to use

D. Sets the controller action to redirect to another page

Solution

1. Step 1: Understand the colon in the route path

   The colon before id means this part of the URL is a variable, called a route parameter.

2. Step 2: Role of route parameters in Rails

   This variable is captured from the URL and passed to the controller action for use, like finding a user by ID.

3. Final Answer:

   Captures part of the URL as a variable to use in the controller -> Option A

4. Quick Check:

   Route parameter = URL variable [OK]

Hint: Colon before name means URL part is a variable [OK]

Common Mistakes:

• Thinking route parameters define fixed paths
• Confusing route parameters with HTTP methods
• Assuming route parameters redirect to other pages

2. Which of the following is the correct syntax to define a route with a parameter named post_id in Rails?

easy

A. get '/posts/post_id', to: 'posts#show'

B. get '/posts/{post_id}', to: 'posts#show'

C. get '/posts/<post_id>', to: 'posts#show'

D. get '/posts/:post_id', to: 'posts#show'

Solution

1. Step 1: Identify correct route parameter syntax

   In Rails, route parameters use a colon before the name, like :post_id.

2. Step 2: Compare options

   Only get '/posts/:post_id', to: 'posts#show' uses the colon syntax correctly. Others use incorrect braces or no colon.

3. Final Answer:

   get '/posts/:post_id', to: 'posts#show' -> Option D

4. Quick Check:

   Colon before parameter name = correct syntax [OK]

Hint: Use colon before parameter name in route path [OK]

Common Mistakes:

• Using curly braces or angle brackets instead of colon
• Omitting colon before parameter name
• Confusing parameter syntax with query strings

3. Given this route:
get '/products/:id', to: 'products#show'
And this controller code:

def show
  @product_id = params[:id]
end

What will @product_id be if the URL is /products/42?

medium

A. The string '42'

B. The integer 42

C. nil

D. An error because params[:id] is undefined

Solution

1. Step 1: Understand route parameter capture

   The :id in the route captures the URL part '42' as a string in params[:id].

2. Step 2: Assigning params value in controller

   The controller assigns @product_id = params[:id], so @product_id is the string '42'. Rails does not convert it automatically to integer.

3. Final Answer:

   The string '42' -> Option A

4. Quick Check:

   Route params are strings by default [OK]

Hint: Route params are strings unless converted [OK]

Common Mistakes:

• Assuming params[:id] is an integer automatically
• Expecting nil if parameter is present
• Thinking params[:id] causes an error if defined in route

4. Identify the error in this route definition:

get '/orders/:order_id/items/:item_id', to: 'orders#item'

Controller action:

def item
  @order = Order.find(params[:id])
  @item = Item.find(params[:item_id])
end

medium

A. Route path should not have two parameters

B. params[:id] is incorrect; should be params[:order_id]

C. Controller action name should be 'show' not 'item'

D. Item.find should use params[:id] instead of params[:item_id]

Solution

1. Step 1: Check route parameters names

   The route defines parameters :order_id and :item_id, so params keys are :order_id and :item_id.

2. Step 2: Check controller parameter usage

   The controller incorrectly uses params[:id] which does not exist. It should use params[:order_id] to find the order.

3. Final Answer:

   params[:id] is incorrect; should be params[:order_id] -> Option B

4. Quick Check:

   Parameter names must match route keys [OK]

Hint: Match params keys exactly to route parameter names [OK]

Common Mistakes:

• Using generic :id when route uses :order_id
• Thinking multiple parameters are not allowed
• Confusing controller action names with route parameters

5. You want to create a nested route to show a comment belonging to a post using route parameters. Which route definition correctly captures both post_id and id for the comment?

hard

A. get '/posts/:id/comments/:post_id', to: 'comments#show'

B. get '/posts/comments/:post_id/:id', to: 'comments#show'

C. get '/posts/:post_id/comments/:id', to: 'comments#show'

D. get '/comments/:id/posts/:post_id', to: 'comments#show'

Solution

1. Step 1: Understand nested resource URL structure

   Nested routes usually put the parent resource first, then the child. Here, post_id comes before id for comment.

2. Step 2: Check route parameter order and names

   get '/posts/:post_id/comments/:id', to: 'comments#show' correctly uses /posts/:post_id/comments/:id capturing post's ID and comment's ID in order.

3. Final Answer:

   get '/posts/:post_id/comments/:id', to: 'comments#show' -> Option C

4. Quick Check:

   Parent ID first, child ID second in nested routes [OK]

Hint: Parent resource param before child in nested routes [OK]

Common Mistakes:

• Swapping order of post_id and comment id
• Putting parameters in wrong positions
• Using incorrect parameter names