class Book
  include Mongoid::Document
  
  field :name
  has_many :pages
end

class Page
  include Mongoid::Document
  belongs_to :book
  
  field :text
end

book = Book.create(:name => "A Tale of Two Cities")
book.pages.build(:text => "It was the best of times, it was the worst of times.")
book.pages.length 
# 1 
book.pages.count
# 0
book.pages[0].save
# true
book.pages.length
# 0
book.pages
# []
book.pages.count
# 1