Mocking and stubbing in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is mocking in Ruby testing?

Mocking is creating fake objects that simulate real objects' behavior to test interactions without using the real objects.

beginner

What does stubbing mean in Ruby tests?

Stubbing means replacing a method with a fake one that returns a specific value, so the test does not depend on the real method's behavior.

beginner

How do you create a stub for a method in RSpec?

Use allow(object).to receive(:method).and_return(value) to make the method return the value during tests.

intermediate

What is the difference between a mock and a stub?

A stub provides canned responses to method calls, while a mock also verifies that certain methods were called with expected arguments.

beginner

Why use mocking and stubbing in tests?

They isolate the code being tested by replacing dependencies, making tests faster, more reliable, and focused on specific behavior.

Which RSpec method is used to stub a method?

A mock(object).method

B allow(object).to receive(:method).and_return(value)

C expect(object).to receive(:method)

D stub(object).method

In RSpec, allow(object).to receive(:method).and_return(value) is used to stub a method.

What does a mock object do besides stubbing methods?

A Verifies that methods are called with expected arguments

B Runs the real method implementation

C Deletes the original object

D Automatically fixes bugs

Mocks verify that certain methods are called with expected arguments during tests.

Why is stubbing useful in tests?

A To replace slow or unreliable methods with fixed return values

B To make tests run slower

C To delete code

D To create real database records

Stubbing replaces methods with fixed return values to make tests faster and more reliable.

Which of these is NOT a benefit of mocking and stubbing?

A Isolating the code under test

B Controlling test environment

C Speeding up tests

D Making tests dependent on external services

Mocking and stubbing reduce dependency on external services, not increase it.

In RSpec, how do you check that a method was called on a mock?

A object.method

B allow(object).to receive(:method)

C expect(object).to receive(:method)

D stub(object).method

Use expect(object).to receive(:method) to verify a method call on a mock.

Explain in your own words what mocking and stubbing are and why they are useful in Ruby testing.

Think about how you can replace parts of your code to test only what you want.

Describe how you would stub a method in RSpec and how you would verify a method call using a mock.

Remember the keywords 'allow' and 'expect' in RSpec.

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