I've got 3 Models:

.../models/book/work.rb

class Book::Work

include Mongoid::Document
include Mongoid::Timestamps

embedded_in :category, class_name: '::Book::Work::Category', inverse_of: :book_works
embedded_in :package, class_name: '::Book::Package', inverse_of: :works

field :name_full, type: String

end

---

.../models/book/work/category.rb

class Book::Work::Category

include Mongoid::Document
include Mongoid::Timestamps

field :name, type: String

embeds_many :book_works, class_name: '::Book::Work', inverse_of: :categories

end

____
..../models/book/package.rb

class Book::Package
include Mongoid::Document
include Mongoid::Timestamps

field :name_short, type: String
field :name_full, type: String
field :name_abbr, type: String
field :name_market, type: String
field :instruction, type: String
field :cost, type: Float

embeds_many :works, class_name: '::Book::Work', inverse_of: :package

end

---

What I do: rails console:

Book::Category.create(name:'Category')
Book::Category.last.works.create(name_full:'Work')
work = Book::Category.last.works.last
Book::Package.create(name_full:'Package')
package = Book::Package.last
package.works << work
=

{The work I've put}

package.save
Book::Package.last.works
= []
But if I do this twice:
" package.works << work "
it will add this work once to my Book::Package.last.works. Also when I do like this:
" package.works << Book::Work.new(name_full: 'Work') "

then it works.

I use Rails 4.1.8, 4.2.0; Mongoid 4.0.0, 4.0.2; Ruby 2.15, 2.2.0; Mongod 2.6.6, 2.6.7, 2.4.13, 3.0 - all the same..