Retrieving array of ids in Mongoid

Question

how do you retrieve an array of IDs in Mongoid?

arr=[\"id1\",\"id2\"]
User.where(:id=>arr)

You can do this eas

Answer 1

user_ids = User.only(:_id).where(:foo => :bar).map(&:_id)
Post.where(:user_id.in => user_ids)

The solution above works fine when amount of users is small. But it will require a lot of memory while there are thousands of users.

User.only(:_id).where(:foo => :bar).map(&:_id)

will create a list of User objects with nil in each field except id.

The solution (for mongoid 2.5):

User.collection.master.where(:foo => :bar).to_a.map {|o| o['_id']}