Route parameters in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are route parameters in Rails?

Route parameters are dynamic parts of a URL that Rails captures and passes to controller actions as variables. They let you handle URLs with changing values, like user IDs.

beginner

How do you define a route with a parameter in Rails?

You add a colon before a name in the route path, like get '/users/:id', to: 'users#show'. The :id is the route parameter.

beginner

How do you access route parameters inside a Rails controller?

You use the params hash. For example, params[:id] gives you the value of the :id parameter from the URL.

intermediate

What happens if a route parameter is missing in the URL?

If the route expects a parameter but it is missing, Rails will not match that route and usually returns a 404 error because the URL does not fit the route pattern.

intermediate

Can route parameters be used for multiple values in Rails?

Yes, you can define multiple parameters like /posts/:post_id/comments/:id. Each parameter is accessible via params[:post_id] and params[:id].

In Rails, how do you define a route that accepts a user ID as a parameter?

A get '/users/*id', to: 'users#show'

B get '/users/:id', to: 'users#show'

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

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

The colon before 'id' defines a route parameter named 'id'.

How do you access the route parameter 'id' inside a Rails controller?

A params.id

B request.id

C params[:id]

D session[:id]

Route parameters are accessed via the params hash using symbols, like params[:id].

What will happen if you visit '/users/' but the route expects '/users/:id'?

A Rails will redirect to '/users/1'

B Rails will match the route and set id to nil

C Rails will show the users list

D Rails will return a 404 error

The route requires an :id parameter, so missing it causes no match and a 404 error.

Can you have multiple route parameters in a single Rails route?

A Yes, like '/posts/:post_id/comments/:id'

B No, only one parameter is allowed

C Yes, but only if they are separated by commas

D No, you must use query strings for multiple values

Rails supports multiple parameters in routes by adding multiple colon-prefixed names.

Which of these is NOT a valid route parameter name in Rails?

A :123param

B :user_id

C :id

D :post_id

Parameter names must start with a letter or underscore, not a number.

Explain how route parameters work in Rails and how you use them in a controller.

Think about how URLs can change and how Rails captures those changes.

Describe what happens when a route parameter is missing in a URL that expects it.

Consider how Rails matches URLs to routes.

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