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

Question

I'm still getting the hang of MongoDB and Mongoid and have hit this snag.

Let's say I have a User that has_and_belongs_to_many items and of course an Item that has_and_belongs_to_many users.

I'd like to be able to count the Users with any Items.

this question recommended adding a scope so I tried adding a scope to User such as

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

but User.count - User.has_no_items.count returns 0.

I've looked at .with_size but that's specific to Array fields.

What is the correct way to do this other than

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

which works but it doesn't seem very elegant.

How do I do this efficiently?

Answer 1

The following works for me with 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" do
    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 that this helps.