Rspec Mocks

RSpec Mocks

RSpec Mocks provides test doubles for isolating code under test from external dependencies.

Test Double Types

Basic Doubles

Creating Doubles

# Anonymous double
user = double

# Named double (better error messages)
user = double("user")

# Double with stubs
user = double("user", name: "John", email: "john@example.com")

Stubbing Methods

user = double("user")
allow(user).to receive(:name).and_return("John")
allow(user).to receive(:save).and_return(true)

# Multiple stubs at once
allow(user).to receive_messages(name: "John", email: "john@example.com")

Return Values

allow(service).to receive(:call).and_return("result")

# Return different values on consecutive calls
allow(service).to receive(:call).and_return(1, 2, 3)
# First call returns 1, second returns 2, third+ returns 3

# Return value from block
allow(service).to receive(:call) { |arg| arg.upcase }

# Raise error
allow(service).to receive(:call).and_raise(StandardError, "error message")
allow(service).to receive(:call).and_raise(CustomError.new("message"))

# Throw symbol
allow(service).to receive(:call).and_throw(:abort)

# Yield to block
allow(service).to receive(:call).and_yield("value")
allow(service).to receive(:call).and_yield(1).and_yield(2)

# Call original implementation (partial doubles)
allow(service).to receive(:call).and_call_original

Verifying Doubles

Verifying doubles check that stubbed methods exist on the real class. Always prefer verifying doubles over plain doubles.

# instance_double - verifies against instance methods
user = instance_double(User)
allow(user).to receive(:name).and_return("John")
allow(user).to receive(:nonexistent)  # Raises error!

# class_double - verifies against class methods
UserService = class_double(UserService)
allow(UserService).to receive(:find).and_return(user)

# object_double - verifies against specific object
original_user = User.new
user = object_double(original_user, name: "John")

Verifying Double Benefits

# If User class changes and removes `name` method:
# - Plain double: Tests pass but code is broken
# - Verifying double: Tests fail, alerting to the issue

user = instance_double(User)
allow(user).to receive(:fullname)  # Typo! Raises:
# User does not implement #fullname

Null Object Doubles

Return nil for unstubbed methods instead of raising:

user = instance_double(User).as_null_object
user.anything  # Returns nil instead of error

Message Expectations

expect vs allow

# allow - Stub without requiring call (test setup)
allow(service).to receive(:call)

# expect - Must be called or test fails (behavior verification)
expect(service).to receive(:call)

Verifying Calls

# Must be called
expect(mailer).to receive(:send_email)

# Must be called with specific arguments
expect(mailer).to receive(:send_email).with("user@example.com", "Welcome!")

# Must be called specific number of times
expect(mailer).to receive(:send_email).once
expect(mailer).to receive(:send_email).twice
expect(mailer).to receive(:send_email).exactly(3).times
expect(mailer).to receive(:send_email).at_least(:once)
expect(mailer).to receive(:send_email).at_most(5).times

# Must not be called
expect(mailer).not_to receive(:send_spam)

Argument Matchers

expect(service).to receive(:call).with("exact value")
expect(service).to receive(:call).with(anything)
expect(service).to receive(:call).with(any_args)
expect(service).to receive(:call).with(no_args)

# Type matching
expect(service).to receive(:call).with(instance_of(User))
expect(service).to receive(:call).with(kind_of(Numeric))

# Pattern matching
expect(service).to receive(:call).with(/pattern/)
expect(service).to receive(:call).with(hash_including(key: "value"))
expect(service).to receive(:call).with(array_including(1, 2))

# Custom matching
expect(service).to receive(:call).with(satisfy { |arg| arg.valid? })

# Combining matchers
expect(service).to receive(:process).with(
  instance_of(User),
  hash_including(notify: true)
)

Spies

Spies verify calls after they happen (more natural test flow):

# Setup: allow the call
mailer = instance_double(Mailer)
allow(mailer).to receive(:send_email)

# Exercise: run the code
user_service = UserService.new(mailer)
user_service.register(user)

# Verify: check it was called
expect(mailer).to have_received(:send_email).with(user.email)

Spy vs Mock Style

# Mock style (expect before action)
expect(mailer).to receive(:send_email)
user_service.register(user)

# Spy style (verify after action) - often clearer
allow(mailer).to receive(:send_email)
user_service.register(user)
expect(mailer).to have_received(:send_email)

Spy Helpers

# Create a spy that tracks all calls
user = spy("user")
user.name
user.email
user.save

expect(user).to have_received(:name)
expect(user).to have_received(:save)

# Verifying spy
user = instance_spy(User)

Partial Doubles

Stub methods on real objects:

user = User.new(name: "John")
allow(user).to receive(:premium?).and_return(true)

user.name       # Returns "John" (real method)
user.premium?   # Returns true (stubbed)

Class Method Stubbing

allow(User).to receive(:find).and_return(user)
allow(Time).to receive(:now).and_return(frozen_time)
allow(ENV).to receive(:[]).with("API_KEY").and_return("test-key")

Dangerous: Stubbing Any Instance

# Avoid when possible - makes tests brittle
allow_any_instance_of(User).to receive(:premium?).and_return(true)
expect_any_instance_of(User).to receive(:save)

Better alternative: Dependency injection

# Instead of stubbing any instance
class UserService
  def initialize(user_class: User)
    @user_class = user_class
  end

  def create(attrs)
    @user_class.new(attrs)
  end
end

# Test with injected double
user_class = class_double(User)
service = UserService.new(user_class: user_class)

Ordering

Enforce call order:

expect(logger).to receive(:start).ordered
expect(processor).to receive(:process).ordered
expect(logger).to receive(:finish).ordered

Configuration

# spec/spec_helper.rb
RSpec.configure do |config|
  config.mock_with :rspec do |mocks|
    # Verify partial doubles against real methods
    mocks.verify_partial_doubles = true

    # Verify doubles in before/after hooks
    mocks.verify_doubled_constant_names = true
  end
end

Best Practices

Use Verifying Doubles

# Good - catches interface changes
user = instance_double(User, name: "John")

# Avoid - doesn't verify interface
user = double("user", name: "John")

Prefer Spies for Verification

# Good - arrange, act, assert order
allow(mailer).to receive(:send)
service.process
expect(mailer).to have_received(:send)

# Harder to read - expect before action
expect(mailer).to receive(:send)
service.process

Don't Over-Mock

# Too much mocking - testing implementation
allow(user).to receive(:first_name).and_return("John")
allow(user).to receive(:last_name).and_return("Doe")
expect(user.full_name).to eq("John Doe")  # Just testing string concat

# Better - test real behavior
user = build(:user, first_name: "John", last_name: "Doe")
expect(user.full_name).to eq("John Doe")

Mock at Boundaries

Mock external services, not internal collaborators:

# Good - mocking external HTTP
allow(HTTPClient).to receive(:get).and_return(response)

# Questionable - mocking internal service
allow(UserValidator).to receive(:validate)  # Maybe just use real one?

Additional Resources

• references/mock-patterns.md - Common mocking patterns