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

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this route constraint match?

Given this Rails route with a constraint, what URL will match the route and what will be the value of params[:id]?

Ruby on Rails

Rails.application.routes.draw do
  get '/items/:id', to: 'items#show', constraints: { id: /^\d{3}$/ }
end

A URL '/items/123' matches, params[:id] = '123'

B URL '/items/12a' matches, params[:id] = '12a'

C URL '/items/1234' matches, params[:id] = '1234'

D URL '/items/abc' matches, params[:id] = 'abc'

💡 Hint

The constraint uses a regular expression to limit the :id parameter to exactly three digits.

The route constraint /^\d{3}$/ means the :id must be exactly three digits. So '/items/123' matches and sets params[:id] to '123'. Other options either have letters or wrong digit counts, so they don't match.

📝 Syntax

intermediate

Which route constraint syntax is correct?

Identify the correct way to apply a route constraint that only allows lowercase letters for the :username parameter.

A get '/users/:username', to: 'users#show', constraints: { username: /[a-z]+/ }

B get '/users/:username', to: 'users#show', constraints: username: '[a-z]+'

C get '/users/:username', to: 'users#show', constraints: { username: '[a-z]+' }

D get '/users/:username', to: 'users#show', constraints: { username: /[A-Z]+/ }

💡 Hint

Constraints expect a Regexp object, not a string.

Option A correctly uses a Ruby regular expression /[a-z]+/ inside a hash for constraints. Option A is missing braces and uses a string instead of Regexp. Option A uses a string instead of Regexp. Option A uses uppercase letters which is incorrect for lowercase only.

🔧 Debug

advanced

Why does this route constraint not work as expected?

Consider this route:

get '/products/:code', to: 'products#show', constraints: { code: /\d{4}/ }

Why does the URL '/products/12345' still match this route?

A The constraint only checks the start of the string, so '12345' matches because it starts with '1234'.

B Rails ignores regex constraints longer than 3 digits.

C The regex matches any string containing four digits, so '12345' matches because it has '1234' inside.

D The regex is not anchored, so it matches any substring with four digits, allowing longer strings.

💡 Hint

Regex without anchors can match substrings anywhere in the string.

The regex /\d{4}/ matches any substring of four digits anywhere in the string. Since '12345' contains '1234' at the start, it matches. To fix this, anchor the regex with ^\d{4}$ to match exactly four digits.

❓ state_output

advanced

What is the value of params[:year] after this route matches?

Given this route:

get '/archive/:year', to: 'archive#show', constraints: { year: /\d{4}/ }

And the URL '/archive/2023', what is the value of params[:year] inside the controller?

A 2023 (integer)

B 'year'

C '2023'

D nil

💡 Hint

Route parameters are always strings.

Route parameters in Rails are always strings. Even if the constraint matches digits, params[:year] will be the string '2023'.

🧠 Conceptual

expert

Which statement about route constraints is true?

Select the correct statement about how route constraints work in Rails.

A Route constraints only accept regular expressions and cannot use custom Ruby objects.

B Route constraints can use a class with a matches? method to apply complex logic.

C Route constraints are evaluated after the controller action is called.

D Route constraints can modify the params hash before the controller receives it.

💡 Hint

Think about how you can customize route matching beyond regex.

Rails allows route constraints to be a class or object that implements a matches?(request) method. This lets you write complex logic to decide if a route matches. Constraints are checked before the controller action runs. They cannot modify params directly.

Practice

1. What is the main purpose of using route constraints in a Rails application?

easy

A. To style the HTML output of views

B. To speed up database queries automatically

C. To manage user sessions and cookies

D. To limit which URLs match routes based on specific patterns or conditions

Solution

1. Step 1: Understand what route constraints do

   Route constraints restrict which URLs can match a route by applying conditions like regex or subdomain checks.

2. Step 2: Compare options to the purpose of constraints

   Only To limit which URLs match routes based on specific patterns or conditions describes limiting URL matches based on patterns or conditions, which is the core use of route constraints.

3. Final Answer:

   To limit which URLs match routes based on specific patterns or conditions -> Option D

4. Quick Check:

   Route constraints = limit URL matches [OK]

Hint: Route constraints control which URLs reach your routes [OK]

Common Mistakes:

• Confusing route constraints with database or view logic
• Thinking constraints manage sessions or cookies
• Assuming constraints style HTML output

2. Which of the following is the correct syntax to add a regex constraint for a route parameter :id that only allows digits?

easy

A. get '/items/:id', to: 'items#show', constraints: { id: /\d+/ }

B. get '/items/:id', to: 'items#show', constraints: { id: '[a-z]+' }

C. get '/items/:id', to: 'items#show', constraints: { id: /[a-z]+/ }

D. get '/items/:id', to: 'items#show', constraints: { id: /\w+/ }

Solution

1. Step 1: Identify regex for digits only

   The regex /\d+/ matches one or more digits, which fits the requirement.

2. Step 2: Check syntax for constraints hash

   The syntax constraints: { id: /\d+/ } correctly applies the regex constraint to the :id parameter.

3. Final Answer:

   get '/items/:id', to: 'items#show', constraints: { id: /\d+/ } -> Option A

4. Quick Check:

   Regex for digits = /\d+/ [OK]

Hint: Use /\d+/ regex for digit-only route params [OK]

Common Mistakes:

• Using string instead of regex for constraints
• Using wrong regex like /[a-z]+/ for digits
• Missing double backslash escape in regex

3. Given the route definition below, what URL will match the route?

get '/profile/:username', to: 'users#show', constraints: { username: /[a-z]{3,}/ }

medium

A. /profile/alex

B. /profile/Jo

C. /profile/123

D. /profile/Alice

Solution

1. Step 1: Understand the regex constraint

   The regex /[a-z]{3,}/ means the username must be at least 3 lowercase letters.

2. Step 2: Check each URL against the regex

   "/profile/Jo" fails because "Jo" has uppercase and only 2 letters.
   "/profile/alex" matches because "alex" is lowercase and 4 letters.
   "/profile/123" fails because digits are not allowed.
   "/profile/Alice" fails because of uppercase 'A'.

3. Final Answer:

   /profile/alex -> Option A

4. Quick Check:

   Regex requires lowercase 3+ letters = /profile/alex/ [OK]

Hint: Match regex carefully: lowercase letters only, length matters [OK]

Common Mistakes:

• Ignoring case sensitivity in regex
• Assuming digits match letter regex
• Not counting minimum length in regex

4. Identify the error in this route constraint code snippet:

get '/orders/:order_id', to: 'orders#show', constraints: { order_id: '\\d+' }

medium

A. The route path is missing a leading slash

B. The controller action is incorrect

C. The regex is a string, it should be a Regexp object

D. The constraints hash key should be a symbol, not a string

Solution

1. Step 1: Check the constraints value type

   The value '\\d+' is a string, but Rails expects a Regexp object for regex constraints.

2. Step 2: Confirm correct regex syntax

   It should be /\d+/ (a Regexp literal), not a string '\\d+'.

3. Final Answer:

   The regex is a string, it should be a Regexp object -> Option C

4. Quick Check:

   Regex constraints need Regexp objects, not strings [OK]

Hint: Use /regex/ not 'string' for route constraints [OK]

Common Mistakes:

• Using string instead of Regexp for constraints
• Confusing route path syntax
• Misnaming controller or action

5. You want to restrict a route to only respond when the subdomain is 'admin'. Which of the following route definitions correctly applies this constraint?

hard

A. get '/dashboard', to: 'admin#dashboard', constraints: { subdomain: 'admin' }

B. get '/dashboard', to: 'admin#dashboard', constraints: lambda { |req| req.subdomain == 'admin' }

C. get '/dashboard', to: 'admin#dashboard', constraints: { domain: 'admin' }

D. get '/dashboard', to: 'admin#dashboard', constraints: { subdomain: /admin/ }

Solution

1. Step 1: Understand subdomain constraints options

   Rails allows constraints as regex, hash, or custom objects like lambdas for complex logic.

2. Step 2: Evaluate each option

   get '/dashboard', to: 'admin#dashboard', constraints: { subdomain: 'admin' } uses a string for subdomain, which is not valid.
   get '/dashboard', to: 'admin#dashboard', constraints: { subdomain: /admin/ } uses regex /admin/, which matches subdomain 'admin' but may match partials.
   get '/dashboard', to: 'admin#dashboard', constraints: lambda { |req| req.subdomain == 'admin' } uses a lambda checking if request subdomain equals 'admin', which is precise and recommended.
   get '/dashboard', to: 'admin#dashboard', constraints: { domain: 'admin' } uses 'domain' key, which is incorrect for subdomain constraints.

3. Final Answer:

   get '/dashboard', to: 'admin#dashboard', constraints: lambda { |req| req.subdomain == 'admin' } -> Option B

4. Quick Check:

   Use lambda for precise subdomain check [OK]

Hint: Use lambda for exact subdomain constraints [OK]

Common Mistakes:

• Using string instead of lambda or regex for subdomain
• Confusing domain with subdomain key
• Using regex that matches partial subdomains unintentionally