Sequel::Model.plugin(:polymorphic)

class Asset < Sequel::Model
  many_to_one :attachable, :polymorphic => true
end

class Note < Sequel::Model
  one_to_many :assets, :as => :attachable
end

class Post < Sequel::Model
  one_to_many :assets, :as => :attachable
end

Sequel.migration do
  change do
    create_table :assets do
      # ...
      Integer :attachable_id
      String :attachable_type
      # ...
      index [:attachable_id, :attachable_type]
    end
  end
end