Test doubles concept in Ruby - Mini Project: Build & Apply

Test doubles concept

📖 Scenario: You are writing tests for a simple Ruby class that sends messages. Sometimes, you want to test your code without actually sending messages. To do this, you use a test double, which acts like a fake message sender.

🎯 Goal: Learn how to create and use a test double in Ruby to simulate a message sender in tests.

📋 What You'll Learn

Create a test double named fake_sender

Set up the test double to respond to send_message method

Call send_message on the test double with a specific message

Print the result of calling send_message on the test double

💡 Why This Matters

🌍 Real World

Test doubles help developers test parts of their code without relying on real objects, which might be slow, unavailable, or have side effects.

💼 Career

Understanding test doubles is important for writing effective automated tests in software development jobs, improving code quality and reliability.

1

Create a test double named fake_sender

Write a line of Ruby code to create a test double called fake_sender using double.

Ruby

# Create a test double named fake_sender
# Your code here

Hint

Use double('Name') to create a test double in Ruby.

2

Set up fake_sender to respond to send_message

Write a line of Ruby code to make fake_sender respond to send_message with the string 'Message sent'.

Ruby

fake_sender = double('MessageSender')
# Make fake_sender respond to send_message
# Your code here

Hint

Use allow(double).to receive(:method).and_return(value) to set the response of a test double method.

3

Call send_message on fake_sender with a message

Write a line of Ruby code to call send_message on fake_sender with the argument 'Hello!' and save the result in a variable called result.

Ruby

fake_sender = double('MessageSender')
allow(fake_sender).to receive(:send_message).and_return('Message sent')
# Call send_message on fake_sender with 'Hello!'
# Your code here

Hint

Call the method on the test double and assign the return value to result.

4

Print the result of calling send_message

Write a line of Ruby code to print the value of the variable result.

Ruby

fake_sender = double('MessageSender')
allow(fake_sender).to receive(:send_message).and_return('Message sent')
result = fake_sender.send_message('Hello!')
# Print the result
# Your code here

Hint

Use puts result to print the value.

Practice

1.

What is the main purpose of using test doubles in Ruby testing?

easy

A. To permanently change the behavior of classes

B. To speed up the program execution in production

C. To replace real objects with fake ones to isolate tests

D. To automatically generate user interface elements

Solution

1. Step 1: Understand what test doubles do

   Test doubles are fake objects used in tests to replace real parts of the program.

2. Step 2: Identify the main goal

   The goal is to isolate the part being tested by removing outside effects from real objects.

3. Final Answer:

   To replace real objects with fake ones to isolate tests -> Option C

4. Quick Check:

   Test doubles isolate tests = A [OK]

Hint: Test doubles fake objects to isolate tests quickly [OK]

Common Mistakes:

• Thinking test doubles speed up production code
• Believing test doubles permanently change classes
• Confusing test doubles with UI tools

2.

Which of the following is the correct way to create a test double named user in Ruby using RSpec?

?

easy

A. user = fake('user')

B. user = double('user')

C. user = stub('user')

D. user = mock_object('user')

Solution

1. Step 1: Recall RSpec syntax for test doubles

   RSpec uses the method double to create test doubles.

2. Step 2: Match the correct method call

   The correct syntax is double('name'), so double('user') is right.

3. Final Answer:

   user = double('user') -> Option B

4. Quick Check:

   RSpec double creation = A [OK]

Hint: Use double('name') to create test doubles in RSpec [OK]

Common Mistakes:

• Using fake or stub instead of double
• Confusing mock_object with double
• Missing quotes around the name

3.

What will be the output of this Ruby code using a test double?

user = double('user')
allow(user).to receive(:name).and_return('Alice')
puts user.name

medium

A. Alice

B. nil

C. user

D. NoMethodError

Solution

1. Step 1: Understand the double setup

   The double user is told to respond to name with 'Alice'.

2. Step 2: Check the method call output

   Calling user.name returns 'Alice' as set by and_return.

3. Final Answer:

   Alice -> Option A

4. Quick Check:

   Double returns stubbed value = C [OK]

Hint: Stub method with and_return to control output [OK]

Common Mistakes:

• Expecting the double name instead of stubbed return
• Thinking it returns nil by default
• Assuming NoMethodError without stub

4.

Identify the error in this test double usage and how to fix it:

order = double('order')
allow(order).to receive(:total)
puts order.total

medium

A. Missing and_return causes order.total to return nil, fix by adding and_return

B. Double name should be a symbol, fix by using double(:order)

C. Cannot call allow on a double, fix by using expect

D. Missing puts before order.total

Solution

1. Step 1: Analyze the stub setup

   The method total is allowed but no return value is set, so it returns nil.

2. Step 2: Fix by adding return value

   Add and_return(value) to specify what total should return.

3. Final Answer:

   Missing and_return causes order.total to return nil, fix by adding and_return -> Option A

4. Quick Check:

   Stub needs return value = B [OK]

Hint: Always add and_return to stub methods for expected output [OK]

Common Mistakes:

• Thinking double name must be symbol
• Confusing allow with expect usage
• Forgetting to print output

5.

You want to test a PaymentProcessor class that calls an external Gateway service. How can you use a test double to ensure your test does not call the real Gateway?

class PaymentProcessor
  def initialize(gateway)
    @gateway = gateway
  end

  def pay(amount)
    @gateway.charge(amount)
  end
end

# Test code here?

hard

A. Remove the gateway parameter and hardcode the call inside PaymentProcessor

B. Call the real Gateway class directly in the test

C. Use allow_any_instance_of(Gateway) to stub charge

D. Create a double for gateway that expects charge with amount and returns true

Solution

1. Step 1: Understand dependency injection

   The PaymentProcessor takes a gateway object to call charge.

2. Step 2: Use a test double for gateway

   Create a double that expects charge with the amount and returns true to simulate success.

3. Step 3: Avoid calling real external service

   This prevents the test from calling the real Gateway, isolating the test.

4. Final Answer:

   Create a double for gateway that expects charge with amount and returns true -> Option D

5. Quick Check:

   Use doubles to isolate external calls = D [OK]

Hint: Inject doubles to replace external services in tests [OK]

Common Mistakes:

• Calling real external services in tests
• Hardcoding dependencies inside classes
• Using allow_any_instance_of unnecessarily