Mongoid 3.0, Paperclip, embedded, accepts_nested_attributes_for and allow_destroy with optional description

_form.html.haml

-# JS stuff here is quick and dirty. Good programmer will put all link_to_function stuff into helpers 
-# like described here (http://railscasts.com/episodes/197-nested-model-form-part-2)
-# or even will do js stuff in some unobtrusive way

= form_for @some_model, :html => {:multipart => true} do |f|
  = f.fields_for :photos do |photo|
       .photo-field
         - if photo.object.new_record?
           -# This stuff is for new fields from form builder
           = photo.text_field :description
           = photo.file_field :file
           = link_to_function "Remove", "$(this).parents('.photo-field').remove();"
         - else
           -# This stuff is for existing photos
           = photo.text_field :description
           = photo.object.file_file_name
           = photo.hidden_field :_destroy, :class => 'destroy'
           = link_to_function "Remove", "$(this).parents('.photo-field').hide(); $(this).siblings('.destroy').val('1')"

     -# You should generate the Remove link here too, but there is to much creepy inline js here so I omitted it
     = link_to_function "Add", "id = new Date().getTime(); $(this).parent().append('<input id=\"some_model_photos_attributes_' + id +'_description\" name=\"some_model[photos_attributes][' + id + '][description]\" type=\"text\"> <input id=\"some_model_photos_attributes_' + id +'_file\" name=\"some_model[photos_attributes][' + id + '][file]\" type=\"file\">');"

Gemfile

gem 'rails', '3.2.1'
gem 'mongoid', :git => 'git://github.com/mongoid/mongoid.git'
gem 'mongoid-paperclip', :require => "mongoid_paperclip"

# if you're using rspec (should be temporary until mongoid-rspec will be updated to mongoid 3.0)
# group :test do
#   gem "mongoid-rspec", :git => 'https://github.com/tanordheim/mongoid-rspec.git', :branch => 'mongoid_3_0'
# end

photo.rb

class Photo
  include Mongoid::Document
  include Mongoid::Paperclip

  field :description, :type => String
  has_mongoid_attached_file :file

  # I use polymorphic relations because I need Photo to be embedded in many models
  # It's not required to achieve working Paperclip
  embedded_in :paparazzi, :polymorphic => true

  attr_accessible :description, :file

  # For reasons unknown Paperclip callbacks prevent parent model from destroying when
  # they called by Mongoid's cascade_callbacks, so we will delete attachments by self
  skip_callback(:destroy, :before, :prepare_for_destroy)
  skip_callback(:destroy, :after, :destroy_attached_files)

  set_callback(:destroy, :after) do |photo|
    photo.file.destroy
  end
end

some_model.rb

class SomeModel
  include Mongoid::Document

  # :cascade_callbacks never mentioned in documentation for mongoid-paperclip
  # but it's obviously required to get photos working as far as file processing
  # done by Paperclip callbacks 
  embeds_many :photos, :cascade_callbacks => true, :as => :paparazzi

  # this kind of reject_if allows user to set empty description, but will filter
  # file fields with no file selected 
  accepts_nested_attributes_for :photos, :allow_destroy => true,
     :reject_if => lambda { |photo| photo[:file].blank? if not photo[:_destroy] }
  attr_accessible :photos_attributes
end

Very helpful, thanks a lot!

mongoid 4 not work for _destroy