Ruby on Rails - Scopes in Rails

Introduction

Scopes in Rails are a way to encapsulate common queries into reusable methods. They help to keep your code DRY (Don't Repeat Yourself) and make your queries more readable and maintainable. This guide will cover how to create and use scopes in Rails models.

Defining Scopes

Scopes are defined in the model using the scope method. Here is a simple example:

# app/models/article.rb
class Article < ApplicationRecord
  scope :published, -> { where(published: true) }
end
                

In this example, the published scope returns all articles where the published attribute is true.

Using Scopes

Once defined, you can use scopes in your queries just like any other method. Here is an example:

# Fetch all published articles
published_articles = Article.published
                

Chaining Scopes

Scopes can be chained together to create more complex queries. Here is an example:

# app/models/article.rb
class Article < ApplicationRecord
  scope :published, -> { where(published: true) }
  scope :recent, -> { order(created_at: :desc) }
end

# Fetch all published articles, ordered by most recent
recent_published_articles = Article.published.recent
                

Scopes with Parameters

Scopes can also accept parameters to make them more flexible. Here is an example:

# app/models/article.rb
class Article < ApplicationRecord
  scope :by_author, ->(author_id) { where(author_id: author_id) }
end

# Fetch all articles by a specific author
author_articles = Article.by_author(1)
                

Combining Scopes with Class Methods

While scopes are great for simple queries, more complex logic can be encapsulated in class methods. Here is an example:

# app/models/article.rb
class Article < ApplicationRecord
  scope :published, -> { where(published: true) }

  def self.recent_published
    published.order(created_at: :desc)
  end
end

# Fetch all published articles, ordered by most recent
recent_published_articles = Article.recent_published
                

Default Scopes

Default scopes are used to apply a scope to all queries for a model. Use them with caution as they can have unexpected side effects. Here is an example:

# app/models/article.rb
class Article < ApplicationRecord
  default_scope { where(active: true) }
end

# Fetch all active articles
active_articles = Article.all
                

In this example, the default_scope ensures that only articles with active: true are returned by default.

Conclusion

Scopes in Rails are a powerful way to encapsulate common queries and make your code more readable and maintainable. By using scopes, you can keep your code DRY and create flexible, reusable query methods for your models.