Error -- File as attribute in a document embedded

[senekis]

Example

    class Assignment
        embeds_many :deliveries
    end

    class Delivery
        include MongoidExt::Storage

        file_key :file
        embedded_in :assignment, :inverse_of => :deliveries
    end

when I create deliveries

    def create
        @delivery = Delivery.new(:flie => params[:assignment][:file])
        @assignment.deliveries << @delivery
        .....
    end

Err -->
Mongoid::Errors::InvalidCollection in DeliveriesController#create

Access to the collection for Delivery is not allowed since it is an embedded document, please access a collection from the root document.

In file mongoid_ext/file.rb in line 52.
def grid_filename
@grid_filename ||= "#{_root_document.collection.name}/#{self.id}"
end