Chained Polymorphic Association (Many-to-Many) not working on ActiveRecord append

I have a chained polymorphic association on two many-to-many relationships.
I am using Ruby 1.8.7
I am using Rails 2.3.4
I am using ActiveRecord 2.3.4
I have the following relationship:

GROUP
  |
  >- GROUPABILITY (polymorphic)
  |
 TAG
  |
  >- TAGABILITY (polymorphic)
  |
CONTACT

The idea is a CONTACT can have many TAGS and a TAG can belong to many GROUPS. Because I plan to use both TAGS and GROUPS in more creative ways is why I want them to be a polymorphic association.

Models

class Group < ActiveRecord::Base
  has_many    :groupabilities
  has_many    :tags, :through => :groupabilities,
              :source => :tag,
              :conditions => "groupabilities.groupable_type = 'Tag'"  
end

class Groupability < ActiveRecord::Base
  belongs_to  :group
  belongs_to  :groupable, :polymorphic => true
  
  belongs_to  :tag, :class_name => "Tag",
        :foreign_key => "groupable_id"
end

class Tag < ActiveRecord::Base
  has_many    :groupabilities, :as => :groupable
  has_many    :groups, :through => :groupabilities
  
  has_many    :tagabilities
  has_many    :contacts, :through => :tagabilities,
              :source => :contact,
              :conditions => "tagabilities.tagable_type = 'Contact'"
end

class Tagability < ActiveRecord::Base
  belongs_to  :tag
  belongs_to  :tagable, :polymorphic => true
  
  belongs_to  :contact, :class_name => "Contact",
        :foreign_key => "tagable_id"
  
end

class Contact < ActiveRecord::Base
  has_many    :tagabilities, :as => :tagable
  has_many    :tags, :through => :tagabilities
end

DB

class CreateGroups < ActiveRecord::Migration
  def self.up
    create_table :groups do |t|
      t.string :name
      t.timestamps
    end
  end

  def self.down
    drop_table :groups
  end
end

class CreateTags < ActiveRecord::Migration
  def self.up
    create_table :tags do |t|
      t.string :name
      t.timestamps
    end
  end

  def self.down
    drop_table :tags
  end
end

class CreateContacts < ActiveRecord::Migration
  def self.up
    create_table :contacts do |t|
      t.string :name
      t.timestamps
    end
  end

  def self.down
    drop_table :contacts
  end
end

class CreateTagabilities < ActiveRecord::Migration
  def self.up
    create_table :tagabilities do |t|
      t.integer   :tagable_id
      t.string    :tagable_type
      t.integer   :tag_id
      t.timestamps
    end
  end

  def self.down
    drop_table :tagabilities
  end
end

class CreateGroupabilities < ActiveRecord::Migration
  def self.up
    create_table :groupabilities do |t|
      t.integer   :groupable_id
      t.string    :groupable_type
      t.integer   :group_id
      t.timestamps
    end
  end

  def self.down
    drop_table :groupabilities
  end
end

Console

Loading development environment (Rails 2.3.2)
>> c = Contact.create(:name => "First Contact")
=> #<Contact id: 1, name: "First Contact", created_at: "2009-09-25 13:05:06", updated_at: "2009-09-25 13:05:06">

>> t = Tag.create(:name => "First Tag")
=> #<Tag id: 1, name: "First Tag", created_at: "2009-09-25 13:05:31", updated_at: "2009-09-25 13:05:31">

>> g = Group.create(:name => "First Group")
=> #<Group id: 1, name: "First Group", created_at: "2009-09-25 13:05:55", updated_at: "2009-09-25 13:05:55">

>> c.tags
=> []

>> c.tags << t
=> [#<Tag id: 1, name: "First Tag", created_at: "2009-09-25 13:05:31", updated_at: "2009-09-25 13:05:31">]

>> c.tags
=> [#<Tag id: 1, name: "First Tag", created_at: "2009-09-25 13:05:31", updated_at: "2009-09-25 13:05:31">]

>> t.contacts
=> [#<Contact id: 1, name: "First Contact", created_at: "2009-09-25 13:05:06", updated_at: "2009-09-25 13:05:06">]

>> g.tags
=> []

>> t.groups
=> []

>> g.tags << t
=> [#<Tag id: 1, name: "First Tag", created_at: "2009-09-25 13:05:31", updated_at: "2009-09-25 13:05:31">]

>> g.tags
=> [#<Tag id: 1, name: "First Tag", created_at: "2009-09-25 13:05:31", updated_at: "2009-09-25 13:05:31">]

>> t.groups
=> []

>> Groupability.all
=> [#<Groupability id: 1, groupable_id: 1, groupable_type: nil, group_id: 1, created_at: "2009-09-25 13:10:30", updated_at: "2009-09-25 13:10:30">]

Problem

The problem is that PER THE LAST LINE OF CONSOLE, groupable_type is not being set. It should be set to "Tag". If I manually set the groupable_type to "Tag" everything works beautifully.

I have tried this in a variety of ways but it seems to be related to the fact that this is a chained polymorphic association.