Using foreign key in store_dir

I am new to Ruby/Rails, and I'm writing an app that allows the user to upload mulitple images to attach to a user. The user data is stored in the Entity model, and the picture data (including file_column) is stored in the Picture model, which has a foreign key to the I want to store the pictures in a directory that has the Entity's id as the last directory name, not the picture's id. Base upon what I've read, I have to use a callback procedure in Picture to create the appropriate directory. This is what I came up with:

_*entity.rb*_ class Entity < ActiveRecord::Base     validates_presence_of :lastname, :address1, :zip, :phone

    belongs_to :entitytype     has_and_belongs_to_many :entitysubtypes     belongs_to :state     belongs_to :city     belongs_to :agency     belongs_to :agent     belongs_to :screentemplate     has_many :comments     has_many :pictures end

*_picture.rb_* class Picture < ActiveRecord::Base     belongs_to :entity     file_column :image,         :web_root => "/picture/entity/",         :store_dir => :new_store_dir

    def new_stor_dir         return "/picture/entity/" + + "/"     end

_*entity_controller.rb*_    def upload         @entity = Entity.find(params["id"])         @pictures =[:pictures]) { |pic| pic.attributes = params[:picture][] }         if @entity.update_attributes(params[:entity])             flash[:notice] = "Pictures uploaded."         else             flash[:notice] = "Problem uploading pictures."         end         render :action => 'show', :id => @entity     end

I must be doing something wrong because I get no file name in the database. I suspect that, since it worked fine without the callback label in store_dir, it must be something I'm doing. Any ideas? Any help would be greatly appreciated!