http://vladimir-vg.github.com/2011/03/10/setup-mongoid-and-carrierwave/

gistfile1.rb

config.grid_fs_host = Mongoid.config.master.connection.host

gistfile2.rb

config.grid_fs_host = Mongoid.database.connection.primary_pool.host

gistfile3.rb

class Image
  include Mongoid::Document

  embedded_in :market_item

  mount_uploader :attach, ImageUploader
end

gistfile4.rb

class MarketItem
  include Mongoid::Document

  embeds_many :images, :stored_as => :array

  # BUGFIX
  # next code-line needed for fix bug array described here:
  # http://tinyurl.com/6yntj3r 
  accepts_nested_attributes_for :images
end

gistfile5.erb

<%= form_for @market_item, :html => {:multipart => true} do |f| -%>
  <%= f.fields_for :images do |builder| -%>
    <%= builder.label :attach %>
    <%= builder.file_field 'attach' %>
  <% end -%>

  <%= f.submit "Отправить" %>
<% end -%>

gistfile6.rb

# GET /market_items/new
# GET /market_items/new.xml
def new
  @market_item = MarketItem.new
  3.times { @market_item.images.build }

  respond_to do |format|
    format.html # new.html.erb
    format.xml  { render :xml => @market_item }
  end
end

gistfile7.rb

# POST /market_items
# POST /market_items.xml
def create
  @market_item = MarketItem.new(params[:market_item])
  
  respond_to do |format|
    if @market_item.save

      # BUGFIX
      # Don't know why, but nested images does not saves automaticly
      # bug a bit described here (at end):
      # http://flux88.com/blog/using-carrierwave-with-mongoid/
      @market_item.images.each {|x| x.save!}

      format.html { redirect_to(@market_item,
                                :notice => 'created.') }
      format.xml  { render :xml => @market_item,
                           :status => :created,
                           :location => @market_item }
    else
      format.html { render :action => "new" }
      format.xml  { render :xml => @market_item.errors,
                           :status => :unprocessable_entity }
    end
  end
end