How to return an empty ActiveRecord relation?

Question

If I have a scope with a lambda and it takes an argument, depending on the value of the argument, I might know that there will not be any matches, but I still want to return

Answer 1

It is possible and so that's:

scope :for_users, lambda { |users| users.any? ? where("user_id IN (?)", users.map(&:id).join(',')) : User.none }

http://apidock.com/rails/v4.0.2/ActiveRecord/QueryMethods/none

Correct me if I'm wrong.

Answer 2

I think I prefer the way this looks to the other options:

scope :none, limit(0)

Leading to something like this:

scope :users, lambda { |ids| ids.present? ? where("user_id IN (?)", ids) : limit(0) }

Answer 3

There are also variants, but all of these are making request to db

where('false')
where('null')