mongo_test.rb

require 'mongoid'

Mongoid.configure do |config|
  config.master = Mongo::Connection.new.db("so_test")
end

class Client
  include Mongoid::Document
  belongs_to :contact
  field :name, type: String
end

class Contact
  include Mongoid::Document
  has_many :clients
  field :name, type: String
end

Contact.create(:name => "Bill")
jill = Contact.create(:name => "Jill")
jill.clients.create(:name => "Steve")

p "Has Clients"
Contact.any_in(_id: Client.all.distinct("contact_id")).each do |c| 
  p c 
end
p "No Clients"
Contact.not_in(_id: Client.all.distinct("contact_id")).each do |c| 
  p c 
end