I’m sure you know about the Law of Demeter. It’s also known as the principle of least knowledge. Let’s see how to apply it in Ruby on Rails !
If you don’t know about it, let me explain it quicly. The law of Demeter is a programming guideline implying that an entity should only talk to its close friends. If we put in another way, an object should not call methods through another object.
When using Rails and the cool associations feature, it can be tempting to chain method 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
You get the idea ! The models will grow out of control if 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
Note 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
Now, we don’t break the law of Demeter anymore since we can access the street directly from the company object. Plus, less BS inside our models ;) Note how we added
allow_nil to the delegate. With this little addition, if the company does not have any building associated, we will just get
nil when calling
street and no exception!
If you never used the delegate method, it’s time to add it to your toolbox !
You can get more information about delegate in the documentation.