Rails extending ActiveRecord::Base

Question

I\'ve done some reading about how to extend ActiveRecord:Base class so my models would have some special methods. What is the easy way to extend it (step by step tutorial)?<

Answer 1

With Rails 4, the concept of using concerns to modularize and DRY up your models has been in highlights.

Concerns basically allow you to group similar code of a model or across multiple models in a single module and then use this module in the models. Here is a example:

Consider a Article model, a Event model and a Comment Model. A article or A 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 Model. Using concerns we can extract this common code in a separate module Commentable.

For this create a commentable.rb file in app/model/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

One point I will like to highlight while using Concerns is that Concerns should be used for 'domain based' grouping rather than 'technical' grouping. For example, a domain grouping is like 'Commentable', 'Taggable' etc. A technical based grouping will be like 'FinderMethods', 'ValidationMethods'.

Here is a link to a post that I found very useful for understanding concerns in Models.

Hope the writeup helps :)