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Mocking, Stubbing & Test Doubles

Test doubles help isolate units, simulate external systems, and verify collaborations without hitting real services. Ruby offers flexible options that work across RSpec and Minitest.

Catalog of doubles

Mocks

Expect calls ahead of time (`expect(service).to receive(:call)`). Use sparingly; prefer spies to avoid overspecifying behaviour.

Stubs

Override method return values during the test (`allow(user).to receive(:timezone).and_return('UTC')`). Restore the original behaviour automatically after each example.

Spies

Observe interactions after the fact (`analytics = spy('Analytics'); ...; expect(analytics).to have_received(:track)`), great for verifying messaging behaviour.

RSpec syntax for doubles

        RSpec.describe LessonPublisher do
          let(:mailer) { instance_double(LessonMailer) }
          let(:analytics) { class_double(AnalyticsService) }

          subject(:publisher) { described_class.new(mailer: mailer, analytics: analytics) }

          before do
            allow(mailer).to receive_message_chain(:published_email, :deliver_later)
            allow(analytics).to receive(:track)
          end

          it 'notifies and tracks the publication' do
            publisher.call(lesson)

            expect(mailer).to have_received(:published_email).with(lesson)
            expect(analytics).to have_received(:track).with('lesson_published', hash_including(lesson_id: lesson.id))
          end
        end
      

Minitest stubs & mocks

        require "test_helper"

        class TrialReminderJobTest < ActiveJob::TestCase
          test "enqueues email" do
            reminder = Minitest::Mock.new
            reminder.expect :call, true, [User]

            TrialReminderJob.any_instance.stub :reminder_service, reminder do
              assert_enqueued_with(job: ActionMailer::MailDeliveryJob) do
                TrialReminderJob.perform_now(users(:demo))
              end
            end

            reminder.verify
          end
        end
      

`Minitest::Mock#expect` verifies arguments and call counts; remember to call `verify`. Use `stub` to patch methods temporarily.

External calls & HTTP mocking

WebMock: intercept HTTP requests (`stub_request(:post, 'https://api.example.com')`). Fail the spec if unexpected calls leak through.
VCR: record and replay HTTP responses. Store cassettes in `spec/fixtures/vcr_cassettes` or `test/support/cassettes`.
Fakes: build lightweight in-memory implementations (e.g., `FakePaymentGateway`) for fast tests while preserving behaviours relevant to the suite.

Avoid these pitfalls

Over-mocking locks tests to implementation. Mock only boundaries, not private methods or Ruby core APIs.
Stubbing time globally? Prefer `travel_to` (ActiveSupport) or `freeze_time` to keep tests deterministic.
Remember to clean state between examples—WebMock + VCR cassettes should reset so tests don’t leak data.

Mocking, Stubbing & Test Doubles

Mocking, Stubbing & Test Doubles

Catalog of doubles

Mocks

Stubs

Spies

RSpec syntax for doubles

Minitest stubs & mocks

External calls & HTTP mocking

Avoid these pitfalls

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Mocking, Stubbing & Test Doubles

Mocking, Stubbing & Test Doubles

Catalog of doubles

Mocks

Stubs

Spies

RSpec syntax for doubles

Minitest stubs & mocks

External calls & HTTP mocking

Avoid these pitfalls

Quick Navigation

Related Topics

Video Tutorial