What is scope/named_scope in rails?

The best way to understand about the details is to go to API Documentation.

You'll get the complete details and the ways we can use Scopes.

API Documentation of Scope

scope in active record is like class methods but they return Relation object which means you can call another scope or active record querying method on it.

For example, if you have a Zombie model (zombies table) with below mentioned scope methods,

class Zombie
  scope :rotting, -> { where(rotting: true) }
  scope :fresh, -> { where('age < ?', 25) }
  scope :recent, -> { order(created_at: :desc) }
end

And you call

Zombie.rotting.fresh.recent.limit(3)

It translates to the below in SQL,

select "zombies.*" from "zombies" where "zombies"."rotting" = 't' and (age<20) order by create_at desc limit 3

Example above is based on rails 4 syntax

A scope is a subset of a collection. Sounds complicated? It isn't. Imagine this:

You have Users. Now, some of those Users are subscribed to your newsletter. You marked those who receive a newsletter by adding a field to the Users Database (user.subscribed_to_newsletter = true). Naturally, you sometimes want to get those Users who are subscribed to your newsletter.

You could, of course, always do this:

User.where(subscribed_to_newsletter: true).each do #something

Instead of always writing this you could, however, do something like this.

#File: users.rb
class User < ActiveRecord::Base
  scope :newsletter, where(subscribed_to_newsletter: true)
  #yada yada
end

If you're using Rails 4 or newer, do this instead:

#File: users.rb
class User < ActiveRecord::Base
  scope :newsletter, -> { where(subscribed_to_newsletter: true) }
  #yada yada
end

This allows you to access your subscribers by simply doing this:

User.newsletter.each do #something

This is a very simple example but in general scopes can be very powerful tools to easy your work.

Check out this link: API Description