How to Use Scopes in Ruby on Rails

Scopes are custom queries that you define inside your Rails models with the scope method.

Every scope takes two arguments:

A name, which you use to call this scope in your code
A lambda, which implements the query

It looks like this:

class User < ApplicationRecord
  scope :active, -> { where("status: 0") }
end

As a result of calling a scope, you’ll get an ActiveRecord::Relation object.

Which means you can chain & combine scopes!

Example:

User.active

When To Use Scopes?

Let’s see an example.

def index
  @books = Book.where("LENGTH(title) > 20")
end 

This is an index controller action that wants to display books with titles longer than 20 characters.

It’s fine.

But if you want to use this query in other places, you’re going to have duplicated code.

Duplicated code makes your project harder to maintain.

Let’s move this query into a scope.

Like this:

class Book
  scope :with_long_title, -> { where("LENGTH(title) > 20") }
end

Now our controller action looks like this:

def index
  @books = Book.with_long_title
end

Summary
Good job! As a result of reading this article, you’ve learned how to use Rails scopes in the most effective way.

Don’t forget to put this new knowledge into practice so you can remember how it works.

Thanks for reading. 🙂

I’d love to hear thoughts or comments around this. Feel free to email me at [e-mail] or hit me up on twitter, @ronakabhattrz.

Comments

[Ian bradbury]

Scopes are great but it's worth highlighting, when a scope returns nil, all records are returned. This can cause unexpected consequences - especially when the scope is being used to limit access to data.

[Merdan Durdyyev]

Exactly. Hey Ronak, great coincidence. I was working on something last night. It's good I came across your post. I can scope 'archived'/'inactive' menus, categories and items in my project. Thank you ...

[Ronak Bhatt]

Welcome 😃