Route parameters in Ruby on Rails - Mini Project: Build & Apply

Route parameters

📖 Scenario: You are building a simple Rails app that shows details about books. Each book has a unique ID. You want to create a route that uses this ID in the URL to show the right book.

🎯 Goal: Build a Rails route that uses a route parameter called :id to capture the book's ID from the URL and pass it to the controller.

📋 What You'll Learn

Create a route with a parameter named :id in config/routes.rb

Define a controller action show in BooksController

Use the route parameter params[:id] inside the show action

Add a view file to display the book ID

💡 Why This Matters

🌍 Real World

Web apps often show details for items like products, users, or posts by using route parameters in the URL.

💼 Career

Understanding route parameters is essential for building dynamic web pages and APIs in Rails development jobs.

1

Create the books data hash

Create a hash called BOOKS in app/controllers/books_controller.rb with these exact entries: 1 => 'The Hobbit', 2 => '1984', 3 => 'Pride and Prejudice'.

Ruby on Rails

# Your code here

Hint

Think of BOOKS as a small dictionary that stores book names by their IDs.

2

Add the route with parameter :id

In config/routes.rb, add a route that maps GET /books/:id to the show action of BooksController.

Ruby on Rails

class BooksController  'The Hobbit', 2 => '1984', 3 => 'Pride and Prejudice'}
end

# Your code here

Hint

Use get '/books/:id', to: 'books#show' to capture the :id from the URL.

3

Define the show action using params[:id]

In BooksController, define a show method that sets @book_name to the value from BOOKS using params[:id].to_i.

Ruby on Rails

class BooksController  'The Hobbit', 2 => '1984', 3 => 'Pride and Prejudice'}

  # Your code here
end

Rails.application.routes.draw do
  get '/books/:id', to: 'books#show'
end

Hint

Use params[:id].to_i to convert the route parameter to a number and find the book.

4

Create the show view to display the book name

Create a file app/views/books/show.html.erb with this exact content: <h1>Book: <%= @book_name %></h1>.

Ruby on Rails

class BooksController  'The Hobbit', 2 => '1984', 3 => 'Pride and Prejudice'}

  def show
    @book_name = BOOKS[params[:id].to_i]
  end
end

Rails.application.routes.draw do
  get '/books/:id', to: 'books#show'
end

# Your code here in app/views/books/show.html.erb

Hint

This view shows the book name inside an <h1> heading.

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