ruby
require 'mongoid' # v4.0.2

Mongoid::Config.load_configuration({
  sessions: {
    default: {
      database: 'db',
      hosts: ['localhost:27017']
    }
  }
})

class Person
  include Mongoid::Document
  has_many :pets
end

class Pet
  include Mongoid::Document
  belongs_to :person
end

RSpec.describe Person do
  let(:person) { Person.create! }
  let(:pet_id) { person.pets.first.id }

  before { 5.times { person.pets.create! } }

  # This Passes
  specify 'finding a pet should not change the count of pets' do
    expect { person.pets.find(pet_id) }.not_to change { person.pets.count }
  end

  # This Fails
  specify 'finding a pet should not change the size of the pets array' do
    expect { person.pets.find(pet_id) }.not_to change { person.pets.to_a.size }
  end

  # This Fails
  specify 'finding a pet should not change the count of the pets target' do
    expect { person.pets.find(pet_id) }.not_to change { person.pets.target.count }
  end

  # This Passes
  context 'when `#each` was NOT called on the pets relation' do
    specify 'finding a pet should not change the size of the pets array' do
      person.pets.find(pet_id)
      expect(person.pets.count).to eq(person.pets.to_a.size)
    end
  end

  # This Fails
  context 'when `#each` WAS called a block on the pets relation' do
    before { person.pets.each {} }
    specify 'finding a pet should not change the size of the pets array' do
      person.pets.find(pet_id)
      expect(person.pets.count).to eq(person.pets.to_a.size)
    end
  end
end