app/models/popolo/identifier.rb
module Popolo
# An issued identifier.
class Identifier
include Mongoid::Document
embedded_in :identifiable, polymorphic: true
# An issued identifier, e.g. a DUNS number.
field :identifier, type: String
# An identifier scheme, e.g. DUNS.
field :scheme, type: String
validates_presence_of :identifier
end
end