ruby on rails - How can an ActiveRecord::Relation object call class methods

Question

How can an ActiveRecord::Relation object call class methods?

class Project < ActiveRecord::Base
  has_many :tasks
end

class Task < ActiveRecord::Base
  belongs_to :project

  def self.initial_tasks # class methods
   # here return initial tasks
  end
end

Now we can call :

Project.first.tasks.initial_tasks # how it works

initial_tasks is a class method and we can't call class methods on an object.

Project.first.tasks returns an ActiveRecord::Relation object, so how could it be able to call initial_tasks?

Please explain.

Answer 1

There's not much documentation on the application on class methods to ActiveRecord::Relation objects, but we can understand this behavior by taking a look at how ActiveRecord scopes work.

First, a Rails model scope will return an ActiveRecord::Relation object. From the docs:

Class methods on your model are automatically available on scopes. Assuming the following setup:

class Article < ActiveRecord::Base
  scope :published, -> { where(published: true) }
  scope :featured, -> { where(featured: true) }

  def self.latest_article
    order('published_at desc').first
  end

  def self.titles
    pluck(:title)
  end
end

First, invoking a scope returns an ActiveRecord::Relation object:

Article.published.class
#=> ActiveRecord::Relation

Article.featured.class
#=> ActiveRecord::Relation

Then, you can operate on the ActiveRecord::Relation object using the respective model's class methods:

Article.published.featured.latest_article
Article.featured.titles

It's a bit of a roundabout way of understanding the relationship between class methods and ActiveRecord::Relation, but the gist is this:

1. By definition, model scopes return ActiveRecord::Relation objects
2. By definition, scopes have access to class methods
3. Therefore, ActiveRecord::Relation objects have access to class methods