Paperclip processor to save dimensions of an image

thubmnail_with_dimensions.rb

# Filename: RAILS_ROOT/lib/paperclip_processors/thumbnail_with_dimensions.rb
# Required Configuration in the model
#   has_attached_file :image,
#    :styles => {
#      :thumbnail => {
#        :geometry => "100x100>",
#        :format => :png,
#        :processors => [:thumbnail_with_dimensions],
#        :mystyle => :thumbnail
#      },
#      :some_other_style => {
#        :geometry => "640x480>",
#        :processors => [:thumbnail_with_dimensions],
#        :mystyle => :some_other_style
#      }
#    }
#
# NOTE: Since the name of the style isn't available in the processor we need to pass it in using a configuration variable.
# I've called it :mystyle just to make sure I don't step on anything in the core.
#
# To get a dimension in a view just do:
# <%= @object.image.width(:thumbnail) %> # Gives the width of the thumbnail
# <%= @object.image.height(:thumbnail) %> # Gives the height of the thumbnail
# <%= @object.image.width %> # Gives the width of the original
# <%= @object.image.dimensions %> Will return the dimensions hash


 module Paperclip
  class Attachment

    def width style = :original
      @dimensions = dimensions unless @dimensions
      begin
        Integer(@dimensions[style.to_s]['width'])
      rescue
        # Integer() throws an exception if it can't do it,
        # we'll eat the exception incase any of our items don't have dimensions saved
        return nil
      end
    end

    def height style = :original
      @dimensions = dimensions unless @dimensions
      begin
        Integer(@dimensions[style.to_s]['height'])
      rescue
        # Integer() throws an exception if it can't do it,
        # we'll eat the exception incase any of our items don't have dimensions saved
        return nil
      end
    end

    def dimensions
      unless @dimensions
        dim = instance_read(:dimensions)
        @dimensions = JSON.parse(dim) unless dim.nil?
      end
      @dimensions
    end
  end
end

module Paperclip
  class ThumbnailWithDimensions < Thumbnail

    def initialize file, options = {}, attachment = nil
      super
      @myinstance = attachment
      @mystyle = options[:mystyle]
      @dimensions = attachment.dimensions
    end

    def make
      dst = super
      @dimensions ||= {} # make sure he have a hash
      @dimensions[@mystyle] = Geometry.from_file dst

      # make sure the original dimensions are saved since we don't actually process the original
      @dimensions[:original] ||= @current_geometry

      @myinstance.instance_write(:dimensions, @dimensions.to_json)
      return dst
    end

  end
end

Just added the processor to my app. Everything work fine if i manually call save on my model after a file is uploaded. Trying to reprocess all my attachements calling rake paperclip:refresh CLASS=MyModel I see that that processor is executed but results are not persisted to the database. Please advice what's wrong.