Mocking and stubbing in Ruby

Introduction

Mocking and stubbing help you test your code by pretending parts of it work in a certain way. This makes testing easier and faster.

When you want to test a method without running its real code.

When a method depends on external services like a database or web API.

When you want to check if a method was called with the right inputs.

When you want to control what a method returns during a test.

When you want to avoid slow or unreliable parts during testing.

Syntax

Ruby

require 'minitest/autorun'
require 'mocha/minitest'

class MyTest < Minitest::Test
  def test_example
    obj = mock()
    obj.stubs(:method_name).returns('value')
    assert_equal 'value', obj.method_name
  end
end

mock() creates a fake object.

stubs(:method_name) tells the fake object to pretend the method returns a value.

Examples

This makes obj.greet always return 'Hello!'.

Ruby

obj = mock()
obj.stubs(:greet).returns('Hello!')
puts obj.greet

This checks that add is called with 2 and 3, then returns 5.

Ruby

obj = mock()
obj.expects(:add).with(2, 3).returns(5)
puts obj.add(2, 3)

This makes obj.time return a fixed date and time.

Ruby

obj = mock()
obj.stubs(:time).returns(Time.new(2024, 1, 1))
puts obj.time

Sample Program

This program shows how to use stubs and mocks in Ruby tests. The add method is stubbed to return 10 instead of 5. The multiply method is mocked to expect specific inputs and return 20.

Ruby

require 'minitest/autorun'
require 'mocha/minitest'

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

  def multiply(a, b)
    a * b
  end
end

class CalculatorTest < Minitest::Test
  def test_add_with_stub
    calc = Calculator.new
    calc.stubs(:add).with(2, 3).returns(10)
    result = calc.add(2, 3)
    puts "Stubbed add result: #{result}"
  end

  def test_multiply_with_mock
    calc = mock()
    calc.expects(:multiply).with(4, 5).returns(20)
    result = calc.multiply(4, 5)
    puts "Mocked multiply result: #{result}"
  end
end

CalculatorTest.new('test_add_with_stub').test_add_with_stub
CalculatorTest.new('test_multiply_with_mock').test_multiply_with_mock

Important Notes

Use stubs when you want to replace a method's return value without caring if it was called.

Use expects when you want to check that a method is called with specific arguments.

Remember to require mocha/minitest to use mocking and stubbing with Minitest.

Summary

Mocking and stubbing help test code by faking parts of it.

Stubs replace method results; mocks check method calls.

They make tests faster and more reliable by avoiding real dependencies.

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