ruby on rails delegate

“Ruby on Rails Delegate : Don’t break the Law of Demeter ! ”

Copyright 2016. Jyaasa Technologies. All Right Reserved

http://jyaasa.com

I am Sarbada Nanda JaiswalAssociate Software Engineer at Jyaasa Technologies

Law of demeter

Each unit should have only limited knowledge about other

units: only units "closely" related to the current unit.

Each unit should only talk to its friends; don't talk to

strangers.

Only talk to your immediate friends.

In object-oriented programming

An object should not call methods through another object.

When using Rails and the cool association feature, it can be tempting to chain methods calls like this:

● product.provider.name

● provider.address.city

● company.building.city

These lines violate the Law of Demeter. To fix it, we should change them to be :

● product.provider_name

● provider.address_city #or provider.city

● company.city

This way, it’s much cleaner and we don’t break the law anymore. But

to do that, we need to add a bunch of methods to our models :

class Product < ActiveRecord::Base

belongs_to :provider

def provider_name

provider.name

end

# More methods to access the provider's attributes

end

class Provider < ActiveRecord::Base

has_many :products

# Has a name attribute

end

We define methods for each attribute like this. But Rails has

a solution : Delegate

class Product < ActiveRecord::Base

belongs_to :provider

delegate :name, to: :provider, prefix: true

end

class Provider < ActiveRecord::Base

has_many :products

# Has a name attribute

end

Options:

o :to - Specifies the target object

o :prefix - Prefixes the new method with the target name or a custom prefix

o :allow_nil - if set to true, prevents a NoMethodError to be raisedNote the use of the prefix key. It allows us to use product.provider_name instead of just

product.name since a product would probably have its own name.

Delegate is a really powerful feature that let you clean up your models.

Let’s see another example.

If we have a Company that belongs to a Building, we could do :

class Company < ActiveRecord::Base

belongs_to :building

delegate :street, :city, :country, to: :building, allow_nil: true

end

class Building < ActiveRecord::Base

has_many :companies

attr_accessible :street, :city, :country

end