Question

In my project I need User to be able to both “create/own” its own objects and “track” (or “follow”) objects created by other users. Here is my approach:

class User < ActiveRecord::Base
  has_many :widgets
  has_many :gadgets
  has_many :nuggets

  has_many :tracks

  has_many :tracked_widgets, :as => :trackable, :trackable_type => "Widget"
  has_many :tracked_gadgets, :as => :trackable, :trackable_type => "Gadget"
  has_many :tracked_nuggets, :as => :trackable, :trackable_type => "Nugget"
end

class Track < ActiveRecord::Base
  belongs_to :trackable, polymorphic: true
  belongs_to :user
end

class Widget < ActiveRecord::Base  # Gadget and Nugget classes are the same
  has_many :tracks, :as => :trackable
end

The schema for the tracks table:

  create_table "tracks", :force => true do |t|
    t.integer  "user_id"
    t.integer  "trackable_id"
    t.string   "trackable_type"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  add_index "tracks", ["trackable_id", "user_id"], :name => "index_tracks_on_trackable_id_and_user_id"

With this implementation I get the following error:

Unknown key: trackable_type (ArgumentError)

Looking at the schema, it’s obvious that I don’t have :trackable_type set up as an index, so the error message makes sense. However, in the examples I’ve seen that demonstrate polymorphic relationships, I’ve never seen the ______able_type field set up as an index. This makes me wonder if I’m missing something basic here. I have these questions:

1. Have I somehow incorrectly set up the has_many and :as => :trackable relationships?
2. If not, is there any issue using a string field as an index?
3. If the answer is simply to add the :trackable_type field as an index, what is the best way to do this after the fact? Add a new migration that creates the :trackable_type index?

Answer 1

I finally figured out how to make the polymorphic association work for this case. Not sure I understand why, but the :as option for has_many is not used in some polymorphic cases. Instead you must use a standard :through option and then reestablish the polymorphism with the :source and :source_type options.

In my example above, the has_many statements in the User model the needed to be formatted as follows:

has_many :tracked_widgets, :through => :tracks, :source => :trackable, :source_type => "Widget"
has_many :tracked_gadgets, :through => :tracks, :source => :trackable, :source_type => "Gadget"
has_many :tracked_nuggets, :through => :tracks, :source => :trackable, :source_type => "Nugget"

Hopes this helps anyone trying to address a similar situation.