[Rails] polymorphic associations and STI

If a Problem with a polymorphic association and STI.

class Message < ActiveRecord::Base
belongs_to :source, :polymorphic => true
end

class Letter < Message
end

class Post < ActiveRecord::Base
has_many :messages, :as => :source
end

Post.first.messages works fine
Post.first.letters

Throws "NoMethodError: undefined method `letters' for
#<Post:0xb6671ef0>"

Thanks for your help.

lg
xinu
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