Testing with Minitest and RSpec

Minitest assertions, RSpec describe and it blocks, let and subject, doubles and mocks, and organising a suite that stays fast.

From assertions to examples

# Minitest
require "minitest/autorun"

class PricingTest < Minitest::Test
  def setup
    @pricing = Pricing.new(discount: 0.15)
  end

  def test_applies_volume_discount
    assert_equal 85_000, @pricing.total(cents: 100_000, quantity: 50)
  end

  def test_rejects_negative_quantity
    assert_raises(ArgumentError) { @pricing.total(cents: 100, quantity: -1) }
  end

  def test_returns_nil_for_unknown_sku
    assert_nil @pricing.price_for("nope")
  end
end

# RSpec
RSpec.describe Pricing do
  subject(:pricing) { described_class.new(discount: 0.15) }

  let(:order) { build(:order, quantity: 50) }

  describe "#total" do
    it "applies the volume discount" do
      expect(pricing.total(cents: 100_000, quantity: 50)).to eq(85_000)
    end

    it "raises for a negative quantity" do
      expect { pricing.total(cents: 100, quantity: -1) }.to raise_error(ArgumentError)
    end
  end
end

• let is lazy and memoised per example; let! evaluates eagerly in a before hook. Neither leaks between examples.
• subject is just let(:subject) with a nicer name, and can be referred to implicitly in a one-liner it { is_expected.to be_valid }.
• Prefer build over create when persistence is not part of the behaviour under test - it is often ten times faster.

Doubles, mocks and stubs

RSpec.describe InvoiceSender do
  let(:mailer)  { instance_double(Mailer, deliver: true) }
  let(:clock)   { class_double(Time, now: Time.utc(2026, 1, 1)) }

  subject(:sender) { described_class.new(mailer: mailer, clock: clock) }

  it "sends an invoice with the current date" do
    invoice = sender.call(order_id: 1)

    expect(mailer).to have_received(:deliver).with(hash_including(date: "2026-01-01"))
    expect(invoice.sent_at).to eq(Time.utc(2026, 1, 1))
  end

  it "does not send twice" do
    sender.call(order_id: 1)
    sender.call(order_id: 1)
    expect(mailer).to have_received(:deliver).once
  end
end

# controlling time without a dependency
travel_to(Time.utc(2026, 6, 1)) do
  expect(subscription.renews_on).to eq(Date.new(2026, 7, 1))
end

# freezing randomness so a failure is reproducible
srand(1234)
expect(generator.pick).to eq("gamma")

Keeping the suite useful

• One behaviour per example, described in the present tense: it "rejects a blank title", not it "tests validation".
• Use described_class and subject so renaming a class does not require editing every spec file.
• Share setup with shared_context and shared_examples, but stop before the abstraction hides the scenario a failing test describes.
• Run --format documentation occasionally and read the output as a specification; if it is unreadable, the test names are wrong.
• --profile and --seed reproduce slow and order-dependent failures. Fix order dependence rather than disabling randomisation.

bundle exec rspec --format progress --profile 10
bundle exec rspec spec/models/book_spec.rb:42   # run one example
bundle exec rake test                            # minitest through Rake
COVERAGE=1 bundle exec rspec                     # with simplecov enabled

FAQ

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Last refreshed 2026-09-18.