How to use concerns in Rails 4
The default Rails 4 project generator now creates the directory "concerns" under controllers and models. I have found some explanations about how to use routing concerns, but nothing about controllers or models.
I am pretty sure it has to do with the current "DCI trend" in the community and would like to give it a try.
The question is, how am I supposed to use this feature, is there a convention on how to define the naming / class hierarchy in order to make it work? How can I include a concern in a model or controller?
So I found it out by myself. It is actually a pretty simple but powerful concept. It has to do with code reuse as in the example below. Basically, the idea is to extract common and / or context specific chunks of code in order to clean up the models and avoid them getting too fat and messy.
As an example, I'll put one well known pattern, the taggable pattern:
# app/models/product.rb
class Product
include Taggable
...
end
# app/models/concerns/taggable.rb
# notice that the file name has to match the module name
# (applying Rails conventions for autoloading)
module Taggable
extend ActiveSupport::Concern
included do
has_many :taggings, as: :taggable
has_many :tags, through: :taggings
class_attribute :tag_limit
end
def tags_string
tags.map(&:name).join(', ')
end
def tags_string=(tag_string)
tag_names = tag_string.to_s.split(', ')
tag_names.each do |tag_name|
tags.build(name: tag_name)
end
end
# methods defined here are going to extend the class, not the instance of it
module ClassMethods
def tag_limit(value)
self.tag_limit_value = value
end
end
end
So following the Product sample, you can add Taggable to any class you desire and share its functionality.
This is pretty well explained by DHH:
In Rails 4, we're going to invite programmers to use concerns with the default app/models/concerns and app/controllers/concerns directories that are automatically part of the load path. Together with the ActiveSupport::Concern wrapper, it's just enough support to make this light-weight factoring mechanism shine.
I have been reading about using model concerns to skin-nize fat models as well as DRY up your model codes. Here is an explanation with examples:
1) DRYing up model codes
Consider a Article model, a Event model and a Comment model. An article or an event has many comments. A comment belongs to either Article or Event.
Traditionally, the models may look like this:
Comment Model:
class Comment < ActiveRecord::Base
belongs_to :commentable, polymorphic: true
end
Article Model:
class Article < ActiveRecord::Base
has_many :comments, as: :commentable
def find_first_comment
comments.first(created_at DESC)
end
def self.least_commented
#return the article with least number of comments
end
end
Event Model
class Event < ActiveRecord::Base
has_many :comments, as: :commentable
def find_first_comment
comments.first(created_at DESC)
end
def self.least_commented
#returns the event with least number of comments
end
end
As we can notice, there is a significant piece of code common to both Event and Article. Using concerns we can extract this common code in a separate module Commentable.
For this create a commentable.rb file in app/models/concerns.
module Commentable
extend ActiveSupport::Concern
included do
has_many :comments, as: :commentable
end
# for the given article/event returns the first comment
def find_first_comment
comments.first(created_at DESC)
end
module ClassMethods
def least_commented
#returns the article/event which has the least number of comments
end
end
end
And now your models look like this :
Comment Model:
class Comment < ActiveRecord::Base
belongs_to :commentable, polymorphic: true
end
Article Model:
class Article < ActiveRecord::Base
include Commentable
end
Event Model:
class Event < ActiveRecord::Base
include Commentable
end
2) Skin-nizing Fat Models.
Consider a Event model. A event has many attenders and comments.
Typically, the event model might look like this
class Event < ActiveRecord::Base
has_many :comments
has_many :attenders
def find_first_comment
# for the given article/event returns the first comment
end
def find_comments_with_word(word)
# for the given event returns an array of comments which contain the given word
end
def self.least_commented
# finds the event which has the least number of comments
end
def self.most_attended
# returns the event with most number of attendes
end
def has_attendee(attendee_id)
# returns true if the event has the mentioned attendee
end
end
Models with many associations and otherwise have tendency to accumulate more and more code and become unmanageable. Concerns provide a way to skin-nize fat modules making them more modularized and easy to understand.
The above model can be refactored using concerns as below: Create a attendable.rb and commentable.rb file in app/models/concerns/event folder
attendable.rb
module Attendable
extend ActiveSupport::Concern
included do
has_many :attenders
end
def has_attender(attender_id)
# returns true if the event has the mentioned attendee
end
module ClassMethods
def most_attended
# returns the event with most number of attendes
end
end
end
commentable.rb
module Commentable
extend ActiveSupport::Concern
included do
has_many :comments
end
def find_first_comment
# for the given article/event returns the first comment
end
def find_comments_with_word(word)
# for the given event returns an array of comments which contain the given word
end
module ClassMethods
def least_commented
# finds the event which has the least number of comments
end
end
end
And now using Concerns, your Event model reduces to
class Event < ActiveRecord::Base
include Commentable
include Attendable
end
* While using concerns its advisable to go for 'domain' based grouping rather than 'technical' grouping. Domain Based grouping is like 'Commentable', 'Photoable', 'Attendable'. Technical grouping will mean 'ValidationMethods', 'FinderMethods' etc
It's worth to mention that using concerns is considered bad idea by many.
Some reasons:
include method, there is a whole dependency handling system - way too much complexity for something that's trivial good old Ruby mixin pattern. Concerns are easy way to shoot yourself in the leg, be careful with them.
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