class Base
  include Mongoid::Document
  has_and_belongs_to_many :relateds, inverse_of: nil
end

class Related
  include Mongoid::Document
end

base1 = Base.create
base1.relateds.pluck(:id)

Given base1.related_ids == [], I would not expect base1.relateds.pluck(:id) to result in a database query. Unfortunately, it does:

> MOPED: 127.0.0.1:27017 QUERY database=mongoid_tests collection=relateds selector={"$and"=>[{"_id"=>{"$in"=>[]}}]} flags=[] limit=0 skip=0 batch_size=nil fields=

(0.4566ms)