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Rails: Law of Demeter Confusion
4 answers
I am reading a book called Rails AntiPatterns and they talk about using delegation to to avoid breaking the Law of Demeter. Here is their prime example:
They believe that calling something like this in the controller is bad (and I agree)
@street = @invoice.customer.address.street
Their proposed solution is to do the following:
class Customer
has_one :address
belongs_to :invoice
def street
address.street
end
end
class Invoice
has_one :customer
def customer_street
customer.street
end
end
@street = @invoice.customer_street
They are stating that since you only use one dot, you are not breaking the Law of Demeter here. I think this is incorrect, because you are still going through customer to go through address to get the invoice's street. I primarily got this idea from a blog post I read:
http://www.dan-manges.com/blog/37
In the blog post the prime example is
class Wallet
attr_accessor :cash
end
class Customer
has_one :wallet
# attribute delegation
def cash
@wallet.cash
end
end
class Paperboy
def collect_money(customer, due_amount)
if customer.cash < due_ammount
raise InsufficientFundsError
else
customer.cash -= due_amount
@collected_amount += due_amount
end
end
end
The blog post states that although there is only one dot customer.cash instead of customer.wallet.cash, this code still violates the Law of Demeter.
Now in the Paperboy collect_money method, we don't have two dots, we
just have one in "customer.cash". Has this delegation solved our
problem? Not at all. If we look at the behavior, a paperboy is still
reaching directly into a customer's wallet to get cash out.
EDIT
I completely understand and agree that this is still a violation and I need to create a method in Wallet called withdraw that handles the payment for me and that I should call that method inside the Customer class. What I don't get is that according to this process, my first example still violates the Law of Demeter because Invoice is still reaching directly into Customer to get the street.
Can somebody help me clear the confusion. I have been searching for the past 2 days trying to let this topic sink in, but it is still confusing.
