Spec for Population mixin that counts instances of class and its subclasses

population_spec.rb

class Animal
  include Population
end

class Kangaroo < Animal
end

class CaptainKangaroo < Kangaroo
end

RSpec.describe "Counting populations of classes" do

  it "Counts the population of the top-most class" do
    expect(Animal.population).to eq(0)

    Animal.new

    expect(Animal.population).to eq(1)
    expect(Animal.population).to eq(1)

    Animal.new

    expect(Animal.population).to eq(2)
  end
  
  it "Counts the population of subclasses, also" do
    expect(Animal.population).to eq(0)
    expect(Kangaroo.population).to eq(0)
    expect(CaptainKangaroo.population).to eq(0)

    Kangaroo.new

    expect(Kangaroo.population).to eq(1)
    expect(Animal.population).to eq(1)
    expect(CaptainKangaroo.population).to eq(0)

    Animal.new

    expect(Animal.population).to eq(2)
    expect(Kangaroo.population).to eq(1)
    expect(CaptainKangaroo.population).to eq(0)

    CaptainKangaroo.new

    expect(Animal.population).to eq(3)
    expect(Kangaroo.population).to eq(2)
    expect(CaptainKangaroo.population).to eq(1)
  end

end