Why Mocking and stubbing in Ruby? - Purpose & Use Cases

The Big Idea

What if you could test your code without waiting for slow or unreliable outside services?

The Scenario

Imagine you are testing a program that talks to a bank to check your balance. Every time you run the test, you have to wait for the bank to respond, and sometimes the bank is slow or down.

The Problem

Waiting for the real bank every time makes tests slow and unreliable. If the bank changes or is offline, your tests break even if your code is fine. Manually creating fake responses each time is tiring and messy.

The Solution

Mocking and stubbing let you create pretend versions of the bank that respond instantly and exactly how you want. This way, tests run fast and always behave the same, so you can focus on your code.

Before vs After

✗ Before

response = BankAPI.get_balance(user_id)
if response == expected
  puts 'Test passed'
end

✓ After

bank_mock = Minitest::Mock.new
bank_mock.expect(:get_balance, 100, [user_id])
result = bank_mock.get_balance(user_id)
assert_equal 100, result

What It Enables

It enables fast, reliable tests that don't depend on outside systems and let you safely try different scenarios.

Real Life Example

When building an app that sends emails, you don't want to send real emails every test. Mocking the email service lets you check if your app tries to send emails without actually sending them.

Key Takeaways

Manual testing with real systems is slow and fragile.

Mocking and stubbing create fake parts that behave predictably.

This makes tests faster, stable, and easier to write.

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