class Book
  include Mongoid::Document
  embeds_many :pages
end

class Page
  include Mongoid::Document
  embedded_in :book
  field :text
  field :number, :type => Integer
  field :read, :type => Boolean
end

b = Book.create
b.pages.create(:text => "ABC", :read => true, :number => 1)
b.pages.create(:text => "DEF", :read => true, :number => 2)
b.pages.create(:text => "GEH", :read => false, :number => 3)

b.reload
b.pages

 # => [#<Page _id: 506a03b9dd365a05d6000002, text: "ABC", number: 1, read: true>, #<Page _id: 506a03b9dd365a05d6000003, text: "DEF", number: 2, read: true>, #<Page _id: 506a03b9dd365a05d6000004, text: "GEH", number: 3, read: true>] 
b.pages.where(:read => false).update_all(:read => true)

# Reload the model and check the pages
# The third page will have nil text and number
b.reload
b.pages
 # => [#<Page _id: 506a03b9dd365a05d6000002, text: "ABC", number: 1, read: true>, #<Page _id: 506a03b9dd365a05d6000003, text: "DEF", number: 2, read: true>, #<Page _id: 506a03cddd365a05d6000005, text: nil, number: nil, read: true>]