Why I no longer define ActiveRecord scopes with class methods

In Rails you can define methods that filter query results, commonly referred to as "scopes", by using the scope method from ActiveRecord and by adding a class method to your model.

Using the scope method involves passing it a lambda:

class Post
  scope :published, -> { where(published: true) }
end

This is essentially syntactic sugar for defining a class method:

class Post < ApplicationRecord
  def self.published
    where(published: true)
  end
end

One important difference when using scope

Sometimes you'll want a conditional statement in your scope to ensure they work when their parameters aren't present.

For example:

class Post < ApplicationRecord
  scope :created_before, ->(time) { where("created_at < ?", time) if time.present? }
end

This will behave similarly to a class method like before:

class Post < ApplicationRecord
  def self.created_before(time)
    where("created_at < ?", time) if time.present?
  end
end

However, there is one important difference between the two. Directly from the Rails guides:

A scope will always return an ActiveRecord::Relation object, even if the conditional evaluates to false, whereas a class method, will return nil. This can cause NoMethodError when chaining class methods with conditionals, if any of the conditionals return false.

This is why I have started to write my scope methods using scope. Keeping your ActiveRecord query methods chainable keeps your application flowing.