Association for polymorphic belongs_to of a particular type

note.rb

class Note < ActiveRecord::Base
  # The true polymorphic association
  belongs_to :subject, polymorphic: true

  # The trick to solve this problem
  has_one :self_ref, class_name: self, foreign_key: :id

  has_one :volunteer, through: :self_ref, source: :subject, source_type: Volunteer
  has_one :participation, through: :self_ref, source: :subject, source_type: Participation
end