gistfile1.rb

class Widget < ActiveRecord::Base
  has_many :asset_maps, :as => :owner
  has_many :assets, :through => :asset_maps
  
  ...
end

class AssetMap < ActiveRecord::Base
  belongs_to :owner, :polymorphic => true
  belongs_to :asset
end

class Asset < ActiveRecord::Base
  
  def owners
    asset_maps.collect { |asset_map| asset_map.owner }
  end

  def after_create
    owners.each { |owner| owner.asset_created }
  end
end