Updated 10 December 2021: Rails 7.0 is introducing a new #invert_where method, which has multiple gotchas that can cause unintended side effects, but can also be used to create a more elegant and more efficient solution to this same problem.

Here’s a neat trick in Rails to use the plumbing of ActiveRecord to keep your models DRY and maintainable. Say you have a scope in an ActiveRecord model with an expires_at column, to find records that are still active¹:

class Foobar < ApplicationRecord
  scope :active, -> { where(expires_at: Time.zone.now..) }
end

You also want to be able to find expired records to be able to clean them out of the database. So you may write an inverse scope:

class Foobar < ApplicationRecord
  scope :active,  -> { where(expires_at: Time.zone.now..) }
  scope :expired, -> { where(expires_at: ...Time.zone.now) }
end

But² what if you want “expired” records to still be usable for a brief period to allow them to be refreshed and extended? Both scopes will need to be changed to reflect the new ranges, or you could use a class-level utility method that just returns 30.minutes.ago or whatever buffer you want to use, and call it from both scopes. Even then, if you later have need for another opposing pair of scopes, you’ll still have to keep both of those scopes mirrored manually as well.

Or you can leverage Arel, the magic that underlies ActiveRecord itself³:

class ApplicationRecord < ActiveRecord::Base
  scope :not, ->(scope) { where(scope.arel.constraints.reduce(:and).not) }
end

Your expired scope is now much simpler and more clearly logically coupled to the active scope⁴:

class Foobar < ApplicationRecord
  scope :active,  -> { where(expires_at: Time.zone.now..) }
  scope :expired, -> { self.not(active) }
end

and any other opposing pairs of scopes in your application can use the same not meta-scope in the same way.

SELECT "foobars".*
FROM "foobars"
WHERE "foobars"."expires_at" >= "2020-06-03 05:44:31.319993"
LIMIT 11

SELECT "foobars".*
FROM "foobars"
WHERE NOT ("foobars"."expires_at" >= "2020-06-03 05:44:31.319993")
LIMIT 11