From rails documentation
Using polymorphic associations in combination with single table inheritance (STI) is a little tricky. In order for the associations to work as expected, ensure that you store the base model for the STI models in the type column of the polymorphic association. To continue with the asset example above, suppose there are guest posts and member posts that use the posts table for STI. In this case, there must be a type column in the posts table.
Change your subscription class like this
class Subscription < ActiveRecord::Base
belongs_to :subscriber, polymorphic: true
belongs_to :area
def subscriber_type=(sType)
super(sType.to_s.classify.constantize.base_class.to_s)
end
end