Mongoid has some issues with Ruby's Marshaling of document object instances. One issue we've run into hard is that when a model uses association extensions and those extensions have been accessed in the lifecycle of the instance, then Marshal.dump fails with the document instance.

require 'rubygems'
require 'mongoid'

class Product
include Mongoid::Document
embeds_many :images do
def active
@target.select(&:active?)
end
end
end

class Image
include Mongoid::Document
embedded_in :product, :inverse_of => :images
def active?
true
end
end

p = Product.new

Marshal.dump(p) #=> "\004\bo:\fProduct\n:\016@accessed{\000:\020@attributes{\006\"\b_ido:\023BSON::ObjectId\006:\n@data[\021iRi'i*iRi\001\324i\001\354i\001\236i\030i\001\261i\000i\000i\006:\023@modifications{\000:\034@previous_modifications{\000:\020@new_recordT"

p.images.build

Marshal.dump(p) #=> TypeError: can't dump anonymous class #<Module:0x103511860>