Mocking and stubbing in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a mock object in Ruby.

Ruby

mock_user = double([1])

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

A:user

B"User"

CUser.new

Duser

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of a symbol.

Trying to instantiate a real object instead of a mock.

In Ruby, double(:user) creates a mock object named :user.

2fill in blank

medium

Complete the code to stub the method 'name' to return 'Alice'.

Ruby

allow(mock_user).to receive(:name).and_return([1])

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

A"Alice"

Bname

C:Alice

DAlice

Attempts:

3 left

💡 Hint

Common Mistakes

Using a symbol instead of a string.

Not quoting the return value.

Use a string "Alice" to return the name properly.

3fill in blank

hard

Fix the error in the code to expect the method 'greet' to be called once.

Ruby

expect(mock_user).to [1](:greet).once

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

Areceive_message

Bcall

Cexpect_message

Dreceive

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect matcher names.

Omitting the receive keyword.

The correct syntax is expect(mock_user).to receive(:greet).once.

4fill in blank

hard

Fill both blanks to stub a method 'age' that returns 30 only if called with argument 'true'.

Ruby

allow(mock_user).to receive(:age).with([1]).and_return([2])

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

Atrue

Bfalse

C30

D20

Attempts:

3 left

💡 Hint

Common Mistakes

Using false instead of true.

Returning the wrong number.

The method age is stubbed to return 30 when called with true.

5fill in blank

hard

Fill all three blanks to create a mock, stub 'status' to 'active', and expect 'update' to be called once.

Ruby

mock_account = double([1])
allow(mock_account).to receive(:status).and_return([2])
expect(mock_account).to receive([3]).once

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

A:account

B"active"

C:update

D:status

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong symbols or strings for mock name or return value.

Expecting the wrong method to be called.

This code creates a mock named :account, stubs status to return "active", and expects update to be called once.

Practice

1. What is the main purpose of stubbing in Ruby testing?

easy

A. To check if a method was called with specific arguments

B. To slow down tests by adding delays

C. To run the actual method code during tests

D. To replace a method's return value with a fake one

Solution

1. Step 1: Understand stubbing concept

   Stubbing means replacing a method's output with a fake value to control test behavior.

2. Step 2: Differentiate from mocking

   Mocking checks method calls, but stubbing focuses on return values.

3. Final Answer:

   To replace a method's return value with a fake one -> Option D

4. Quick Check:

   Stubbing = fake return value [OK]

Hint: Stubs fake returns; mocks check calls [OK]

Common Mistakes:

• Confusing stubs with mocks
• Thinking stubs run real code
• Believing stubs slow tests

2. Which of the following is the correct syntax to stub a method foo on an object obj to return 42 using RSpec?

easy

A. allow(obj).to receive(:foo).and_return(42)

B. expect(obj).to stub(:foo).and_return(42)

C. obj.stub(:foo).returns(42)

D. mock(obj).expects(:foo).returns(42)

Solution

1. Step 1: Recall RSpec stubbing syntax

   In RSpec, stubbing uses allow(object).to receive(:method).and_return(value).

2. Step 2: Check each option

   allow(obj).to receive(:foo).and_return(42) matches correct syntax; others use outdated or incorrect methods.

3. Final Answer:

   allow(obj).to receive(:foo).and_return(42) -> Option A

4. Quick Check:

   RSpec stub = allow + receive + and_return [OK]

Hint: Use allow(obj).to receive(:method).and_return(value) [OK]

Common Mistakes:

• Using deprecated stub syntax
• Confusing expect with allow for stubs
• Using mock instead of allow for stubs

3. What will be the output of this Ruby code using RSpec mocks?

class Calculator
  def add(a, b)
    a + b
  end
end

calc = Calculator.new
allow(calc).to receive(:add).with(2, 3).and_return(10)
puts calc.add(2, 3)
puts calc.add(4, 5)

medium

A. 10
10

B. 10
9

C. 5
9

D. 5
5

Solution

1. Step 1: Analyze stubbed method call

   The method add is stubbed to return 10 only when called with arguments 2 and 3.

2. Step 2: Check calls and outputs

   First call calc.add(2, 3) returns 10 due to stub. Second call calc.add(4, 5) is not stubbed, so runs original method returning 9.

3. Final Answer:

   10 9 -> Option B

4. Quick Check:

   Stubbed args return 10; others run real method [OK]

Hint: Stub affects only specified arguments [OK]

Common Mistakes:

• Assuming all calls return stub value
• Ignoring method arguments in stub
• Expecting syntax error

4. Identify the error in this RSpec mocking code:

user = double('User')
expect(user).to receive(:login).with('admin')
user.login('guest')

medium

A. Missing stub for login method

B. The double is not created correctly

C. The method login is called with wrong argument

D. No error, code is correct

Solution

1. Step 1: Understand the expectation

   The test expects login to be called with argument 'admin'.

2. Step 2: Check actual method call

   The method is called with 'guest', which does not match the expectation.

3. Final Answer:

   The method login is called with wrong argument -> Option C

4. Quick Check:

   Expect argument mismatch causes failure [OK]

Hint: Match expected arguments exactly in mocks [OK]

Common Mistakes:

• Calling method with different arguments
• Confusing stub and expect syntax
• Ignoring error raised by RSpec

5. You want to test a method that calls an external API method fetch_data on a Service object. How can you use mocking and stubbing together to test your method without calling the real API?

hard

A. Stub fetch_data to return fake data and mock to check it was called once

B. Call the real fetch_data method and ignore mocking

C. Mock fetch_data without stubbing its return value

D. Do not use mocking or stubbing; test with real API

Solution

1. Step 1: Stub the API call to avoid real network

   Stub fetch_data to return controlled fake data, so tests run fast and reliably.

2. Step 2: Mock to verify the method call

   Use mock to check fetch_data was called exactly once, ensuring your method triggers it.

3. Final Answer:

   Stub fetch_data to return fake data and mock to check it was called once -> Option A

4. Quick Check:

   Stub return + mock call count = test isolation [OK]

Hint: Stub return and mock call count together [OK]

Common Mistakes:

• Calling real API in tests
• Mocking without stubbing return value
• Ignoring call verification