RSpec Testing

RSpec Testing

This skill provides comprehensive guidance for writing effective RSpec tests in Ruby and Rails applications. Use for writing new specs, fixing failing tests, understanding matchers, using test doubles, and following RSpec best practices.

Quick Reference

Basic Structure

RSpec.describe Order do
  subject(:order) { described_class.new(items) }
  let(:items) { [item1, item2] }
  let(:item1) { double("item", price: 10) }
  let(:item2) { double("item", price: 20) }

  describe "#total" do
    it "sums item prices" do
      expect(order.total).to eq(30)
    end
  end

  context "with discount" do
    let(:order) { described_class.new(items, discount: 5) }

    it "applies discount" do
      expect(order.total).to eq(25)
    end
  end
end

Key Concepts

Writing Good Specs

Use Named Subject for Method Tests

describe "#calculate_total" do
  subject(:total) { order.calculate_total }

  it "returns sum of items" do
    expect(total).to eq(100)
  end
end

Context Blocks for Different States

describe "#withdraw" do
  context "with sufficient funds" do
    let(:account) { build(:account, balance: 100) }

    it "reduces balance" do
      expect { account.withdraw(50) }.to change(account, :balance).by(-50)
    end
  end

  context "with insufficient funds" do
    let(:account) { build(:account, balance: 10) }

    it "raises error" do
      expect { account.withdraw(50) }.to raise_error(InsufficientFunds)
    end
  end
end

Common Matchers

Equality

expect(x).to eq(y)          # ==
expect(x).to eql(y)         # eql? (type-sensitive)
expect(x).to be(y)          # equal? (identity)

Truthiness

expect(x).to be_truthy      # not nil or false
expect(x).to be_falsey      # nil or false
expect(x).to be_nil
expect(x).to be true        # exactly true

Comparisons

expect(x).to be > 3
expect(x).to be_between(1, 10).inclusive
expect(x).to be_within(0.1).of(3.14)

Collections

expect(arr).to include(1, 2)
expect(arr).to contain_exactly(3, 2, 1)  # order-independent
expect(arr).to all(be_positive)
expect(str).to start_with("hello")
expect(hash).to have_key(:name)

Changes

expect { x += 1 }.to change { x }.by(1)
expect { x += 1 }.to change { x }.from(0).to(1)
expect { user.save }.to change(User, :count).by(1)

Errors

expect { raise "boom" }.to raise_error
expect { raise ArgumentError, "bad" }.to raise_error(ArgumentError, /bad/)

Predicates (Dynamic)

expect([]).to be_empty      # [].empty?
expect(user).to be_valid    # user.valid?
expect(hash).to have_key(k) # hash.has_key?(k)

Test Doubles

Types

# Basic double (strict)
user = double("user", name: "Bob")

# Verifying doubles (recommended)
user = instance_double("User", name: "Bob")  # validates instance methods
api = class_double("Api", fetch: data)       # validates class methods
logger = object_double(Rails.logger)         # validates object methods

# Spy (null object for after-the-fact verification)
notifier = spy("notifier")

Stubbing

allow(user).to receive(:name).and_return("Bob")
allow(Api).to receive(:fetch).and_return(data)
allow(obj).to receive(:method) { computed_value }

Expectations

expect(user).to receive(:save).and_return(true)
expect(Api).to receive(:post).with(hash_including(id: 1))

# Spy pattern (verify after action)
notifier = spy("notifier")
service.call(notifier)
expect(notifier).to have_received(:notify).with("done")

Argument Matchers

expect(obj).to receive(:call).with(anything)
expect(obj).to receive(:call).with(kind_of(Integer))
expect(obj).to receive(:call).with(hash_including(a: 1))
expect(obj).to receive(:call).with(array_including(1, 2))

Rails Specs

See examples/rails/ for complete spec templates.

Best Practices

Do

• Use described_class instead of hardcoding class name
• Use let for test data, let! when database records must exist before test
• Use named subject when referencing in tests: subject(:user) { ... }
• Use context blocks to organize different scenarios
• Use verifying doubles (instance_double) over plain double
• Name examples with verbs: it "creates user" not it "should create user"
• Keep examples focused on one behavior
• Use factories over fixtures for flexible test data
• Prefer build_stubbed or build over create when database not needed

Don't

• Don't use instance variables (@user) - use let for type safety
• Don't use before just to trigger let evaluation - use let! instead:

  # BAD - before just to initialize
  let(:user) { create(:user) }
  before { user }
  
  # GOOD - let! for eager evaluation
  let!(:user) { create(:user) }
  

  Use before for side-effects like sign_in(user) or driven_by(:rack_test)
• Don't use let! when let suffices (wastes resources)
• Don't test Rails framework (validations work, focus on business logic)
• Don't stub the object under test
• Avoid any_instance_of - prefer stubbing ClassName.new to return a double (see references/mocks.md)
• Don't use receive_message_chain (violates Law of Demeter)
• Don't write examples without descriptions
• Shared examples work best for testing concerns across including classes. For unique behaviors, prefer repetition over abstraction

Factory Bot

# Build strategies
user = build(:user)              # In-memory, not persisted
user = create(:user)             # Persisted to database
user = build_stubbed(:user)      # Fake persisted (fastest)
attrs = attributes_for(:user)    # Hash of attributes

# With traits and attributes
user = create(:user, :admin, :verified, name: "Bob")

# Lists
users = create_list(:user, 5, :admin)

Strategy Selection:

• build_stubbed - Unit tests without database (fastest)
• build - Validation tests, method tests
• create - Database queries, scopes, associations
• attributes_for - Controller params

See references/factory_bot.md for traits, sequences, associations, and callbacks.

Configuration

Essential settings for spec_helper.rb and rails_helper.rb:

See examples/core/configuration.rb for complete setup.

Additional Resources

Reference Files

For detailed patterns and complete API references, consult:

• references/core.md - describe, it, hooks, let/subject, shared examples, anti-patterns
• references/factory_bot.md - Build strategies, traits, sequences, associations, callbacks
• references/matchers.md - All built-in matchers, composing matchers, custom matchers
• references/mocks.md - Test doubles, stubbing, spies, argument matchers
• references/rails.md - All Rails spec types, Rails matchers, request/system specs

Example Files

Ready-to-use code patterns in examples/:

• examples/core/ - Basic structure, hooks, let/subject, shared examples, configuration
• examples/matchers/ - Equality, collections, change, errors, predicates, custom matchers
• examples/mocks/ - Doubles, stubs, spies, argument matchers, any_instance, constants
• examples/rails/ - Model, request, controller, system, job, mailer, routing specs
• examples/factory_bot/ - Build strategies, traits, associations, callbacks, transients

Running Specs

rspec                           # all specs
rspec spec/models               # directory
rspec spec/user_spec.rb         # file
rspec spec/user_spec.rb:23      # line
rspec --format doc              # documentation format
rspec --only-failures           # re-run failures
rspec --profile 10              # show slowest

Debugging

rspec --seed 12345              # reproduce random order
rspec --fail-fast               # stop on first failure
rspec --backtrace               # full backtrace