class Message < ActiveRecord::Base

  acts_as_nested_set :scope => 'discussable_id = #{discussable_id} AND discussable_type = \'#{discussable_type}\''

  belongs_to :discussable, :polymorphic => true

  # ...
end

class Foo < ActiveRecord::Base

  has_many :messages, :as => :discussable

end

class Bar < ActiveRecord::Base

  has_many :messages, :as => :discussable

end