Scopes in Rails 3

19 February 2011
Scopes have become much more useful in Ruby on Rails 3 with the adoption ofArel into ActiveRecord. The first and mostobvious benefit related to scopes is that everything is scoped! That means youno longer have to use find(...) on an ActiveRecord model and have a queryimmediately be executed. You can actually build queries up and only executethem when you use them. For example:
In Rails 2.x the find method worked like this:

User.find(
  :all,
  :joins => :profile,
  :conditions => ['profile.age = ?', 33])

This would then execute the query and find all users with age 33. In Rails 3.xthis is much simpler:

User.joins(:profile).where('profile.age = ?', 33)

Not only is this more readable since you don't have to put your entire queryinto a single function call, but the main difference between these two commandsis that the second doesn't execute a query. You actually have to call .all,.count, .each or .first to get the query to execute. What's great about that?Well, it means you can chain conditions together before you execute them:

query = User.joins(:profile).where('profile.age = ?', 33)
query.where('users.name = ?', name) unless name.nil?
query.where('profile.email = ?', email) unless email.nil?
query.all

This really shines when you combine it with scopes. For instance, if we want tosimplify the above code, we can do the following:

class User
  scope :by_age, lambda do |age|
    joins(:profile).where('profile.age = ?', age) unless age.nil?
  end
  scope :by_name, lambda{ |name| where(name: name) unless name.nil? }
  scope :by_email, lambda do |email|
    joins(:profile).where('profile.email = ?', email) unless email.nil?
  end
end

User.by_age(33).by_name(params[:name]).by_email(params[:email]).all

Isn't that just awesome!? Not only is the code easier to read and understand,but it's re-usable and scoped! Here's one more example:

class Tag
  belongs_to :post
end

class Post
  has_many :tags
  belongs_to :user

  scope :tagged, lambda do |tag|
    joins(:tags).where('tags.name = ?', tag).group('posts.id')
  end
end

class User
  has_many :posts
end

# How many of my posts are tagged with 'ruby-on-rails'?
User.where('users.id = ?', 232423).posts.tagged('ruby-on-rails').count