test_example.rb

require 'rails_helper'

class Person
  attr_reader(
    :first_name,
    :last_name,
    :age,
    :country
  )

  def initialize(first_name:, last_name: nil, age:, country:)
    @first_name = first_name
    @last_name = last_name
    @age = age
    @country = country
  end

  def full_name
    "#{first_name} #{last_name}"
  end

  def can_buy_alco?
    country.can_buy_alco?(self)
  end
end

module Countries
  class USA
    def can_buy_alco?(person)
      person.age >= 21
    end
  end

  class Belarus
    def can_buy_alco?(person)
      person.age >= 18
    end
  end
end

describe Countries::USA do
  let(:country) { described_class.new }
  let(:person) { double }

  before do
    allow(person).to receive(:age).and_return(age)
  end

  context "when person's age < 21" do
    let(:age) { 18 }

    it { expect(country.can_buy_alco?(person)).to be(false) }
  end

  context "when person's age >= 21" do
    let(:age) { 30 }

    it { expect(country.can_buy_alco?(person)).to be(true) }
  end
end

describe Countries::Belarus do
  let(:country) { described_class.new }

  describe "#can_buy_alco?" do

  end
end

describe Person do
  let(:params) do
    {
      first_name: first_name, last_name: last_name, age: age, country: country
    }.compact
  end
  let(:first_name) { "Don" }
  let(:last_name) { nil }
  let(:country) { Countries::USA.new }
  let(:age) { 40 }

  describe "#full_name" do
    subject(:full_name) do
      described_class.new(**params).full_name
    end

    let(:last_name) { nil }
    let(:age) { 50 }

    context "when first name and last name are present" do
      let(:first_name) { "Don" }
      let(:last_name) { "Keeler" }

      it { expect(full_name).to  eq("#{first_name} #{last_name}") }
    end

    context "when last name is not passed" do
      let(:first_name) { "Don" }

      it { expect(full_name).to  eq("#{first_name} #{last_name}") }
    end
  end

  describe '#can_buy_alco?' do
    let(:person) { Person.new(**params) }

    context "when person can buy alco in a country" do
      let(:country) { double }

      before do
        allow(country).to receive(:can_buy_alco?).with(person).and_return(true)
      end

      it do
        expect(person.can_buy_alco?).to be(true)
      end
    end

    context "when person can not buy alco in a country" do
      let(:country) { double }

      before do
        allow(country).to receive(:can_buy_alco?).with(person).and_return(false)
      end

      it do
        expect(person.can_buy_alco?).to be(false)
      end
    end
  end
end