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

Recall & Review

beginner

What is a route constraint in Rails?

A route constraint is a rule that limits which requests match a route. It helps control routing by checking conditions like request parameters, formats, or custom logic before choosing a route.

beginner

How do you apply a simple format constraint to a route in Rails?

You add constraints: { format: 'json' } to the route. This means the route only matches requests asking for JSON format.

intermediate

What is the purpose of a custom constraint class in Rails routing?

A custom constraint class lets you write Ruby code to decide if a route matches. It must have a <code>matches?</code> method that returns true or false based on the request.

intermediate

Example: How to restrict a route to only match if a user is an admin using a constraint?

Create a class like <code>AdminConstraint</code> with a <code>matches?</code> method checking if <code>request.session[:user_role] == 'admin'</code>. Then use <code>constraints AdminConstraint.new do ... end</code> around the route.

beginner

What happens if a route constraint returns false for a request?

Rails skips that route and tries the next one. If no routes match, it returns a 404 Not Found error.

Which of these is a valid way to add a format constraint to a Rails route?

A get '/posts', to: 'posts#index', match: 'json'

B get '/posts', to: 'posts#index', format: 'json'

C get '/posts', to: 'posts#index', constraints: { format: 'json' }

D get '/posts', to: 'posts#index', only: 'json'

Option C correctly uses the constraints hash to limit the route to JSON format.

What method must a custom constraint class implement in Rails?

A call

B matches?

C check

D validate

The custom constraint class must implement the matches? method that returns true or false.

If a route constraint returns false, what does Rails do?

A Raises an error

B Redirects to the homepage

C Stops routing and returns 500

D Skips the route and tries the next one

Rails skips the route and tries the next matching route if the constraint returns false.

Which of these can be used as a route constraint in Rails?

A All of the above

B A custom class with matches? method

C A regular expression

D A hash with key-value pairs

Rails supports regex, custom classes, and hashes as route constraints.

Why use route constraints in a Rails app?

A To restrict routes based on request details

B To improve route matching speed

C To style routes with CSS

D To generate database queries

Route constraints restrict which requests match a route based on conditions like format or user role.

Explain how to create and use a custom route constraint in Rails.

Think about how to check request details before routing.

Describe what happens when a route constraint does not match a request.

Consider the routing process flow.