Polymorphic :through associations

Could someone please help me with this association scheme? I am
familiar with habtm as well as the :through association but this one
has me stumped.

Here is an example DB.

Tags Table:

id
name

Taggables Table:

id
tag_id
taggble_type
taggable_id

For purposes of this example say that there are 2 other tables that
can be tagged and their model name would appear in the taggable_type
field along with their id in the taggable id.

I have seen this done somewhat with the:

belongs_to :taggable, :polymorphic => true

but never with a join table or a :through to store extra info.
Can this be done this way or am I completely off track?

Thanks,
Chris

Maybe something like this would work for you?

http://wiki.rubyonrails.org/rails/pages/ManytoManyPolymorphicAssociations

-ns