demeter_example.rb

# This example is adapted from Dan Manges's blog, dcmanges.com
class Wallet ; attr_accessor :credit_balance ; end
class Moviegoer
  attr_accessor :wallet
  def initialize
    # ...setup wallet attribute with correct credit balance
  end
end
class MovieTheater
  def collect_money(moviegoer, amount)
    # VIOLATION OF DEMETER (see text)
    if moviegoer.wallet.credit_balance < amount
      raise InsufficientFundsError
    else
      moviegoer.wallet.credit_balance -= due_amount
      @collected_amount += due_amount
    end
  end
end
# Imagine testing the above code:
describe MovieTheater do
  describe "collecting money" do
    it "should raise error if moviegoer can't pay" do
      # "Mock trainwreck" is a warning of a Demeter violation
      wallet = mock('wallet', :credit_balance => 5.00)
      moviegoer = mock('moviegoer', :wallet => wallet)
      lambda { @theater.collect_money(moviegoer, 10.00) }.
        should raise_error(...)
    end
  end
end