class Book
  include Mongoid::Document
  attr_accessible :title
  embeds_many :pages
  field :title
end
class Page
  include Mongoid::Document
  attr_accessible :text
  embedded_in :book
  field :text
end

b = Book.create(:title => "Cloud Atlas")
b.pages.create(text: "One")
b.pages.create(text: "Two")
b.pages.create(text: "Three")
b.pages.update_all(:text => "ZERO")
b.pages.all.map(&:text)
b.pages.update_all(:text => "ZERO")

=> #<Book _id: 507ab6341ffc6dde39000005, _type: nil, title: "Cloud Atlas">
=> #<Page _id: 507ab6341ffc6dde39000006, _type: nil, text: "One">
=> #<Page _id: 507ab6341ffc6dde39000007, _type: nil, text: "Two">
=> #<Page _id: 507ab6341ffc6dde39000008, _type: nil, text: "Three">
=> nil
=> ["ZERO", "ZERO", "ZERO"]
=> nil

=> #<Book _id: 507ab7e41ffc6d7f3a000001, _type: nil, title: "Cloud Atlas">
=> #<Page _id: 507ab7e41ffc6d7f3a000002, _type: nil, text: "One">
=> #<Page _id: 507ab7e41ffc6d7f3a000003, _type: nil, text: "Two">
=> #<Page _id: 507ab7e41ffc6d7f3a000004, _type: nil, text: "Three">
=> nil
=> ["ZERO", "ZERO", "ZERO"]
TypeError: can't convert nil into Hash
from /Users/user/.rvm/gems/ruby-1.9.3-p125/gems/mongoid-3.0.9/lib/mongoid/contextual/memory.rb:289:in `merge!'