Ruby Rspec - Testable classes with dependencies

dependency_injection_spec.rb

require 'rspec'
require 'date'

class Person
  
  def can_drink?
    age >= 21
  end

  def age
    date_class.today.year - 1975
  end

  def date_class
    Date
  end

end


describe "Testable classes with dependencies" do

  let(:user){ Person.new }
  let(:fakeDate){ double "Date", today: Date.new(1995) }

  it "Default behaviour" do
    expect(user.can_drink?).to eq true
  end

  it "Method stubbing" do
    # Hook into the date class
    allow(Date).to receive(:today).and_return Date.new(1995)
    expect(user.can_drink?).to eq false
  end

  it "Dependency injection" do
    # Create a fake Date class and stub only the date_class inside Person.
    # On complex scenarios this is usually a better choice.
    allow(user).to receive(:date_class).and_return fakeDate
    expect(user.can_drink?).to eq false
  end

end