I recently hit on the brilliant idea of using STI (single table inheritence), polymorphic classes, nested forms and has_one through models all at once just to prove how silly I am.

My use case is an asset manager backed by paperclip. I wanted all assets to go into a single S3 folder and be mananged by a single :assets table. Then I wanted any other model to be able to associate uploaded assets at any arity. Gumpf!

My base class for an STI compliant asset table is

# == Schema Information
#
# Table name: assets
#
#  id                      :integer         not null, primary key
#  attachment_file_name    :string(255)
#  attachment_content_type :string(255)
#  attachment_file_size    :integer
#  attachment_updated_at   :datetime
#  created_at              :datetime        not null
#  updated_at              :datetime        not null
#  type                    :string(255)
#  attachment_processing   :boolean
#  terminated              :boolean         default(FALSE)
#

    class Asset < ActiveRecord::Base
        belongs_to :assetable, :polymorphic => true
        delegate :url, :to => :attachment
        delegate :expiring_url, :to => :attachment
        delegate :present?, :to => :attachment

        validates_attachment_presence :attachment

        has_many :assetable_assets, :dependent => :destroy
        has_many :assetables, :through => :assetable_assets, :as => :assetable

        AssetPath = ":rails_env/assets/:id/:style.:extension"

        def self.configure_attachment options = {}
            options.merge! :path => AssetPath
            has_attached_file :attachment, options
        end

    end

Then I need a class to associate other objects, assetables, with uploaded assets. Note that assetables are polymorphic

    # == Schema Information
    #
    # Table name: assetable_assets
    #
    #  id             :integer         not null, primary key
    #  assetable_id   :integer
    #  assetable_type :string(255)
    #  asset_id       :integer
    #  created_at     :datetime        not null
    #  updated_at     :datetime        not null
    #

    class AssetableAsset < ActiveRecord::Base
        belongs_to :assetable, :polymorphic => true
        belongs_to :asset

        # Clean up orphans
        before_destroy do
            if not asset.nil?
                if asset.assetable_assets.count == 1
                    asset.background_destroy
                end
            end
        end
    end

And an example of a concrete assetable class.

# == Schema Information
#
# Table name: users
#
#  id                     :integer         not null, primary key
#  email                  :string(255)     default(""), not null
#

class User < ActiveRecord::Base

    has_one :assetable_asset, :as => :assetable, :dependent => :destroy
    has_one :avatar, :source => :asset, :through => :assetable_asset, :class_name => "ImageAsset"
    attr_accessible :avatar_attributes
    accepts_nested_attributes_for :avatar

    def build_avatar attributes={}, opts={}
        asset = ::ImageAsset.new attributes, opts
        build_assetable_asset :asset => asset
        self.avatar = asset
    end

    def avatar_with_build
        a = avatar_without_build
        unless a
            return build_avatar
        end
        a
    end
    alias_method_chain :avatar, :build
end

Now I can upload an avatar via a formtastic form as so

= semantic_form_for @user, :url => user_profile_path(@user), :html => { :multipart => true } do |f|
        = f.inputs do 
            .span5
                = f.input :email
                = f.input :name

                - if user.avater.present?
                    = image_tag user.avatar.url(:thumb)

                = f.inputs :attachment, :for => :avatar do |a|
                    = a.input :attachment, :as => :file
        = f.buttons

For some reason that I can’t get my head around Rails doesn’t ship with a handler for polymorphic has_one builders. I expected to be able to do

    user.build_avatar

out of the box but I got an error that the method does not exist and it took some fiddling to figure out how to emulate it.

For reference, The image asset class is defined below which fleshes out the STI part of the problem. Maybe it would not be necessary to have STI support on the assets table but it might happen that I have assets with different styles and need to classify them as such.

class ImageAsset < Asset

    configure_attachment styles: ImageSizes.standard_sizes_hash

    validates_attachment_content_type :attachment, 
        :content_type => %r{image/.*}, 
        :less_than => 5.megabyte

end