asset.rb

class Asset < ActiveRecord::Base
  belongs_to :owner, :polymorphic => true

  class User < self
    has_attached_file :image, :styles => { :original => "160x160>" }
    before_create :destroy_previous_asset
  end

protected
  def destroy_previous_asset
    unless owner_id.blank?
      obj = self.class.find_by_owner_id_and_owner_type(owner_id, owner_type)
      obj.destroy unless obj.nil?
    end
  end
end

has_asset.rb

module HasAsset
  def has_asset asset_name, options
    has_one(asset_name, { :as => :owner, :dependent => :destroy }.merge(options))

    asset_name = asset_name.to_s
    asset_class = options[:class_name].constantize

    alias_method "#{asset_name}__old", asset_name

    define_method asset_name do
      @asset_vars ||= {}
      send("#{asset_name}__old") || (@asset_vars[asset_name] ||= asset_class.new)
    end

    define_method "#{asset_name}_id=" do |new_value|
      send("#{asset_name}=", asset_class.find(new_value)) unless new_value.blank?
    end

    define_method "#{asset_name}_url" do |style|
      send(asset_name).image.url(style)
    end

    define_method "#{asset_name}?" do
      send(asset_name).image?
    end
  end
end

ActiveRecord::Base.send(:extend, HasAsset) 

user.rb

class User < ActiveRecord::Base
  has_asset :logo, :class_name => "Asset::User"
end