mongodb - How to select all records where related collection is empty with Mongoid -

i'm still getting hang of mongodb , mongoid , have hit snag.

let's have user has_and_belongs_to_many items , of course item has_and_belongs_to_many users.

i'd able count users items.

this question recommended adding scope tried adding scope user such as

scope :has_no_items, where(:items.empty?) 

but user.count - user.has_no_items.count returns 0.

i've looked @ .with_size that's specific array fields.

what correct way other than

count = 0 user.each { |u| count += 1 unless u.items.empty? } 

which works doesn't seem elegant.

how do efficiently?

the following works me rails 3.2.13, mongoid 3.1.4, moped 1.5.0.

app/models/user.rb

class user   include mongoid::document   field :name, type: string   has_and_belongs_to_many :items   scope :has_items, where(:item_ids.ne => nil)   scope :has_no_items, where(:item_ids => nil) end 

app/models/item.rb

class item   include mongoid::document   field :name, type: string   has_and_belongs_to_many :users end 

test/unit/user_test.rb

require 'test_helper'

class usertest < activesupport::testcase   def setup     user.delete_all     item.delete_all   end    test "users without items"     fagin = user.create(:name => 'fagin')     oliver = user.create(:name => 'oliver')     fagin.items << item.create(:name => 'cane')     assert_equal 2, user.count     assert_equal 1, item.count     assert_equal 1, user.has_items.count     assert_equal 1, user.has_no_items.count     puts     puts "user.has_items: #{user.has_items.to_a.inspect}"     puts "user.has_no_items: #{user.has_no_items.to_a.inspect}"   end end 

rake test

run options:  # running tests:  [1/1] usertest#test_users_without_items user.has_items: [#<user _id: 51df00987f11ba3d7d000001, name: "fagin", item_ids: ["51df00987f11ba3d7d000003"]>] user.has_no_items: [#<user _id: 51df00987f11ba3d7d000002, name: "oliver", item_ids: nil>] finished tests in 0.042205s, 23.6939 tests/s, 94.7755 assertions/s. 1 tests, 4 assertions, 0 failures, 0 errors, 0 skips 

hope helps.