In rails console, with identity_map_enabled: false

> Book.find(123).series.books.count
14
> Book.find(123).series.books.to_a.length
14
> Book.find(123).series.books.to_a.length
14

With identity_map_enabled: true

> Book.find(123).series.books.count
14
> Book.find(123).series.books.to_a.length
14
> Book.find(123).series.books.to_a.length
16

My books are reproducing! It looks like one of the books is getting shoved into the second result set multiple times. I'm happy to investigate further if you tell me where to look.

class Book
  include Mongoid::Document
  belongs_to :series
end

class Series
  include Mongoid::Document  
  has_many :books
end