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.