Trying to retrieve :shape in model

gistfile1.rb

class UploadsController < ApplicationController

def create
  ...
  @upload.shape = "photo"
  ...
  if @upload.save
  ...
end


class Upload < ActiveRecord::Base

  has_attached_file :attachment,
    :path => ":class/:parent_type/:id/:style.:extension",
    :styles => lambda { |a|
      
      # Don't apply assign_shape (= do not set style) when content_type == picture
      if ['image/jpeg','image/png','image/gif'].include?(a.instance_read(:content_type))
        a.instance.assign_shape( What to pass in here to get :shape value I've set in @upload.shape] )
      else
        {}
      end
    }

  def assign_shape(shape)
    case shape
    when "gallery"
      { :preview => "700x467>", :mid => "350x234>", :small => "230x153>" , :tiny => "100x67>" }
    when "photo"
      { :preview => "270x346>", :small => "100x128>" , :tiny => "78x100>" }
    when "logo"
      { :preview => "250x250>", :small => "150x150>" , :tiny => "70x70>" }
    when "sponsor"
      { :preview => "250x250>", :super => "728x90>" , :halfsize => "234x60!" }
    when "generic_file"
      {}
    else
      { :standard => "500x500>", :small => "250x250>", :tiny => "125x125>" }  
    end
  end

end