The Law of Demeter

Law of Demeter
Don’t Mess With The Law
‘

@michaelelfassy
Michael Elfassy
Smashing Boxes

Definition
1. Each unit should have only limited knowledge about
other units
2. Each unit should only talk to its friends; don't talk to
strangers.
3. Only talk to your immediate friends.
source: Wikipedia

In other words
An object should only invoke methods of these kind of
objects:
1. itself
2. its parameters
3. any objects it creates
4. its direct component objects
5. objects of the same type*

itself
class User
attr_accessor :first_name, :last_name
def name
"#{first_name} #{last_name}"
end
end

its parameters
def nickname(name)
"#{name[0..2]}o"
end

objects it creates
def total
calculator = Calculator.new
average + calculator.square(mean)
end

direct components
def initialize()
@calculator = Calculator.new
end
def total
average + @calculator.square(mean)
end

objects of the same type*
# String
name.strip.gsub(/^a/,'b').constantize
# ActiveRecord Relation
User.where(...).order

Violations
class User
belongs_to :department
def division_name
department.division.name
end
end

Violations
try().try()
smells bad!
class User
def division_name
# we know the user has a department
dep = department
# we're assuming the department has a function called division
# which returns the associated division object
# we're assuming that every department belongs to a division
div = dep.division
# we're assuming the division has a name attribute
division_name = div.name
end
end

Dependency assumption
● Duplication
o Once you traverse in one place you are likely to traverse in another place
● Few places understand the relationships
● Don’t create “context”
● at each step of the traversal, the value
returned is decided by the object being
asked

Following the Law
class User
def division_name
# ask the department for a division name (you decide how you are going to get it!)
department.try(:division_name)
end
end
class Department
belongs_to :division
def division_name
# ask the division for a name (you decide how you are going to get it!)
division.name
end
end

The rails way(™)
class User
delegate :division_name, to: :department, allow_nil: true
end
class Department
belongs_to :division
delegate :name, to: :division, prefix: true
delegate :director, to: :division
end

Testing
Now we can test models independently without
having to do nested stubs!

I am the law
let’s replace all . with _ !
NO…
Don’t only fix the violations, look at what they
are telling you. Refactor!

Wallet example
class Wallet
attr_accessor :cash
end
class Customer
has_one :wallet
end
class Paperboy
def collect_money(customer, due_amount)
if customer.wallet.cash < due_ammount
raise InsufficientFundsError
else
customer.wallet.cash -= due_amount
@collected_amount += due_amount
end
end
end
paperboy should not be taking cash
out of a customer's wallet

Delegate attributes
class Customer
has_one :wallet
# attribute delegation
def cash
@wallet.cash
end
end
class Paperboy
if customer.cash < due_ammount
raise InsufficientFundsError
else
customer.cash -= due_amount
@collected_amount += due_amount
end
end
end
paperboy doesn’t need to know how much
cash the client has in his wallet

Delegate Behaviour
class Customer
has_one :wallet
# behavior delegation
def pay(amount)
@wallet.withdraw(amount)
end
end
class Paperboy
@collected_amount += customer.pay(due_amount)
end
end
Tell don’t ask

Summary
● It’s a guideline more than a law
● Look for the spirit of the law
● try.try just smells
Hint, it’s not a real gun

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The Law of Demeter

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