How to use concerns in Rails 4 - ruby-on-rails

Question

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?

Answer 1

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.

Answer 2

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

Answer 3

It's worth to mention that using concerns is considered bad idea by many.
like this guy
and this one
Some reasons:
There is some dark magic happening behind the scenes - Concern is patching include method, there is a whole dependency handling system - way too much complexity for something that's trivial good old Ruby mixin pattern.
Your classes are no less dry. If you stuff 50 public methods in various modules and include them, your class still has 50 public methods, it's just that you hide that code smell, sort of put your garbage in the drawers.
Codebase is actually harder to navigate with all those concerns around.
Are you sure all members of your team have same understanding what should really substitute concern?
Concerns are easy way to shoot yourself in the leg, be careful with them.

Answer 4

This post helped me understand concerns.
# app/models/trader.rb
class Trader
include Shared::Schedule
end
# app/models/concerns/shared/schedule.rb
module Shared::Schedule
extend ActiveSupport::Concern
...
end

Answer 5

I felt most of the examples here demonstrated the power of module rather than how ActiveSupport::Concern adds value to module.
Example 1: More readable modules.
So without concerns this how a typical module will be.
module M
def self.included(base)
base.extend ClassMethods
base.class_eval do
scope :disabled, -> { where(disabled: true) }
end
end
def instance_method
...
end
module ClassMethods
...
end
end
After refactoring with ActiveSupport::Concern.
require 'active_support/concern'
module M
extend ActiveSupport::Concern
included do
scope :disabled, -> { where(disabled: true) }
end
class_methods do
...
end
def instance_method
...
end
end
You see instance methods, class methods and included block are less messy. Concerns will inject them appropriately for you. That's one advantage of using ActiveSupport::Concern.
Example 2: Handle module dependencies gracefully.
module Foo
def self.included(base)
base.class_eval do
def self.method_injected_by_foo_to_host_klass
...
end
end
end
end
module Bar
def self.included(base)
base.method_injected_by_foo_to_host_klass
end
end
class Host
include Foo # We need to include this dependency for Bar
include Bar # Bar is the module that Host really needs
end
In this example Bar is the module that Host really needs. But since Bar has dependency with Foo the Host class have to include Foo (but wait why does Host want to know about Foo? Can it be avoided?).
So Bar adds dependency everywhere it goes. And order of inclusion also matters here. This adds lot of complexity/dependency to huge code base.
After refactoring with ActiveSupport::Concern
require 'active_support/concern'
module Foo
extend ActiveSupport::Concern
included do
def self.method_injected_by_foo_to_host_klass
...
end
end
end
module Bar
extend ActiveSupport::Concern
include Foo
included do
self.method_injected_by_foo_to_host_klass
end
end
class Host
include Bar # It works, now Bar takes care of its dependencies
end
Now it looks simple.
If you are thinking why can't we add Foo dependency in Bar module itself? That won't work since method_injected_by_foo_to_host_klass have to be injected in a class that's including Bar not on Bar module itself.
Source: Rails ActiveSupport::Concern

Answer 6

In concerns make file filename.rb
For example I want in my application where attribute create_by exist update there value by 1, and 0 for updated_by
module TestConcern
extend ActiveSupport::Concern
def checkattributes
if self.has_attribute?(:created_by)
self.update_attributes(created_by: 1)
end
if self.has_attribute?(:updated_by)
self.update_attributes(updated_by: 0)
end
end
end
If you want to pass arguments in action
included do
before_action only: [:create] do
blaablaa(options)
end
end
after that include in your model like this:
class Role < ActiveRecord::Base
include TestConcern
end