Polymorphic association question

This is an example from the geokit plugin which I am trying to
reproduce.

class Location < ActiveRecord::Base
  belongs_to :locatable, :polymorphic => true
  acts_as_mappable
end

class Company < ActiveRecord::Base
  has_one :location, :as => :locatable # also works for belongs_to
associations
  acts_as_mappable :through => :location
end

We see two separate models here. Now is there also a third model, like
some kind of join table? Also is there a locatable_id:integer and
locatable_type:string as well?

The polymorphic :locatable association implies a :locatable_type and :locatable_id column. These columns reference a row in an arbitrary table identified by :locatable_type so no join table is needed.