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.