Test doubles concept in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a test double named 'user'.

Ruby

user = [1]("user")

Drag options to blanks, or click blank then click option'

Amock

Bdouble

Cstub

Dfake

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'mock' instead of 'double' which is not a direct method in RSpec.

Confusing 'stub' which is used to replace methods, not create doubles.

In Ruby testing, double creates a test double object.

2fill in blank

medium

Complete the code to make the test double respond to 'name' with 'Alice'.

Ruby

allow(user).to receive(:[1]).and_return("Alice")

Drag options to blanks, or click blank then click option'

Aage

Bid

Cname

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Using an unrelated method name that the double does not respond to.

Confusing the symbol name with a string.

The receive method specifies which message the double should respond to. Here, it is :name.

3fill in blank

hard

Fix the error in the code to verify the double received the 'save' message.

Ruby

expect(user).to have_received(:[1])

Drag options to blanks, or click blank then click option'

Acreate

Bdelete

Cupdate

Dsave

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that was never called on the double.

Confusing the matcher syntax.

The have_received matcher checks if the double received the :save message.

4fill in blank

hard

Fill both blanks to create a double that responds to 'title' with 'Book' and 'author' with 'John'.

Ruby

book = double("book", [1]: "Book", [2]: "John")

Drag options to blanks, or click blank then click option'

Atitle

Bname

Cauthor

Dpublisher

Attempts:

3 left

💡 Hint

Common Mistakes

Using unrelated attribute names.

Mixing up keys and values.

The double is created with attributes title and author set to given values.

5fill in blank

hard

Fill all three blanks to create a double 'car' with 'make' as 'Toyota', 'model' as 'Corolla', and verify it received 'start'.

Ruby

car = double("car", [1]: "Toyota", [2]: "Corolla")
allow(car).to receive(:start)
car.start
expect(car).to have_received(:[3])

Drag options to blanks, or click blank then click option'

Amake

Bmodel

Cstart

Dstop

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong attribute names.

Checking for a method that was not called.

The double has make and model attributes, and we verify it received the start message.

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