The magical document eating has_many assignment (now incorporating the dark arts): https://github.com/mongoid/mongoid/issues/1503

has_many_assignment_spec.rb

require 'bundler'
Bundler.setup
require 'mongoid'
require 'rspec/autorun'

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

class Kid 
  include Mongoid::Document

  belongs_to :parent
end

class Parent
  include Mongoid::Document

  has_many :kids
end

describe "Mongoid parent with has_many mass-assignment" do
  subject     { Parent.create! }
  let(:kids)  { (0..9).map { |i| Kid.create! } }

  before(:each) do
    Kid.destroy_all
    Parent.destroy_all

    subject.kids << kids
  end

  context "without reloading the parent" do
    its("kids.count") { should be == 10 }

    context "after assigning new Kids" do
      before(:each) do
        subject.kids = new_kids
      end
      
      context "from an array of existing elements" do
        let(:new_kids) { Kid.all.to_a } # on_the_block

        its("kids.count") { should be == 10 }
        specify { Kid.count.should be == 10 }
      end

      context "from a collection proxy of existing elements" do
        let(:new_kids) { Kid.all }

        its("kids.count") { should be == 10 }
        specify { Kid.count.should be == 10 }
      end

      context "from an array of new and old elements" do
        let(:new_kids) { [ Kid.create! ] + Kid.all.take(3) + [ Kid.create! ] }

        its("kids.count") { should be == 5 } # Failed: got 2
        specify { Kid.count.should be == 12 }
      end
    end
  end

  context "after reloading the parent" do
    before(:each) do
      subject.reload
    end

    its("kids.count") { should be == 10 }

    context "after assigning new Kids" do
      before(:each) do
        subject.kids = new_kids
      end
      
      context "from an array of existing elements" do
        let(:new_kids) { Kid.all.to_a }

        its("kids.count") { should be == 10 } # Failed: got 0
        specify { Kid.count.should be == 10 }
      end

      context "from a collection proxy of existing elements" do
        let(:new_kids) { Kid.all }

        its("kids.count") { should be == 10 }
        specify { Kid.count.should be == 10 }
      end

      context "from an array of new and old elements" do
        let(:new_kids) { [ Kid.create! ] + Kid.all.take(3) + [ Kid.create! ] }

        its("kids.count") { should be == 5 } # Failed: got 2
        specify { Kid.count.should be == 12 }
      end
    end
  end
end

test.rb

# This is a slightly modified version of the original gist to show a related bug

require 'bundler'
Bundler.setup
require 'mongoid'

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

class Kid 
  include Mongoid::Document
  belongs_to :parent
end
class Parent
  include Mongoid::Document
  has_many :kids
end

Kid.destroy_all
Parent.destroy_all

p = Parent.create!
10.times do |n|
  k = Kid.create!
  p.kids << k
end

p.reload
puts "Parent has #{p.kids.count} kids"

kids = Kid.all.to_a
p = Parent.find(p.id)
p.kids = kids

puts "There are now #{Kid.count} kids"

# New line which shows a different bug from #1173
puts "Parent has #{p.kids.count} kids" # Says 0, should be 10

test_output.sh

➜  mongoid git:(master) ✗ ruby test.rb
Parent has 10 kids
There are now 10 kids
Parent has 0 kids