Mocking and stubbing in Ruby - Time & Space Complexity

Time Complexity: Mocking and stubbing

O(n)

Understanding Time Complexity

When using mocking and stubbing in tests, it's important to understand how the time to run tests changes as the number of mocks grows.

We want to see how the test execution time scales with more mocks or stubs.

Scenario Under Consideration

Analyze the time complexity of the following Ruby test code using mocks and stubs.

    1. require 'minitest/autorun'
    2. class User
    3.   def greet
    4.     "Hello!"
    5.   end
    6. end
    7. 
    8. class UserTest < Minitest::Test
    9.   def test_greet
   10.     user = Minitest::Mock.new
   11.     user.expect(:greet, "Hi!")
   12.     assert_equal "Hi!", user.greet
   13.     user.verify
   14.   end
   15. end
    

This code tests a User class method by replacing it temporarily with a mocked response.

Identify Repeating Operations

Look for repeated actions that affect time.

• Primary operation: Running each test method with its stub setup and teardown.
• How many times: Once per test case, but if many tests or many stubs are used, this repeats for each.

How Execution Grows With Input

Imagine adding more tests or stubs.

Number of Tests (n)	Approx. Operations
10	About 10 stub setups and test runs
100	About 100 stub setups and test runs
1000	About 1000 stub setups and test runs

Pattern observation: The total time grows roughly in direct proportion to the number of tests using mocks or stubs.

Final Time Complexity

Time Complexity: O(n)

This means the test time grows linearly as you add more mocked or stubbed tests.

Common Mistake

[X] Wrong: "Mocking or stubbing makes tests run instantly, so time doesn't grow with more tests."

[OK] Correct: Each mock or stub still takes setup and teardown time, so more tests mean more total time.

Interview Connect

Understanding how mocks and stubs affect test time helps you write efficient tests and explain trade-offs clearly in interviews.

Self-Check

What if we replaced stubs with real method calls? How would the time complexity change?

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