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

  # Same as subject but where subject_type is 'Volunteer'
  belongs_to :volunteer, source_association: :subject
  # Same as subject but where subject_type is 'Participation'
  belongs_to :participation, source_association: :subject
end

class Note < ActiveRecord::Base
  belongs_to :subject, polymorphic: true
  belongs_to :volunteer, class_name: "Volunteer", foreign_key: :subject_id, conditions: {notes: {subject_type: "Volunteer"}}
  belongs_to :participation, class_name: "Participation", foreign_key: :subject_id, conditions: {notes: {subject_type: "Participation"}}
end

describe Note do
  context 'on volunteer' do
    let!(:volunteer) { create(:volunteer) }
    let!(:note) { create(:note, subject: volunteer) }
    let!(:unrelated_note) { create(:note) }

    it 'narrows note scope to volunteer' do
      scoped = Note.scoped
      scoped = scoped.joins(:volunteer).where(volunteers: {id: volunteer.id})
      expect(scoped.count).to be 1
      expect(scoped.first.id).to eq note.id
    end

    it 'allows access to the volunteer' do
      expect(note.volunteer).to eq volunteer
    end

    it 'does not return participation' do
      expect(note.participation).to be_nil
    end

  end
end

1) Note on volunteer allows access to the volunteer
     Failure/Error: expect(note.reload.volunteer).to eq volunteer
     ActiveRecord::StatementInvalid:
       PG::Error: ERROR:  missing FROM-clause entry for table "notes"
       LINE 1: ...."deleted" = 'f' AND "volunteers"."id" = 7798 AND "notes"."s...
                                                                    ^
       : SELECT  "volunteers".* FROM "volunteers"  WHERE "volunteers"."deleted" = 'f' AND "volunteers"."id" = 7798 AND "notes"."subject_type" = 'Volunteer' LIMIT 1
     # ./spec/models/note_spec.rb:10:in `block (3 levels) in <top (required)>'

scope :scoped_by_volunteer_id, lambda { |volunteer_id| where({subject_type: 'Volunteer', subject_id: volunteer_id}) }
  scope :scoped_by_participation_id, lambda { |participation_id| where({subject_type: 'Participation', subject_id: participation_id}) }

class Note
  belongs_to :volunteer,
    ->(note) {where('1 = ?', (note.subject_type == 'Volunteer')},
    :foreign_key => 'subject_id'
end

module Notable
   def self.included(other)
     Note.belongs_to(other.to_s.underscore.to_sym,
       ->(note) {where('1 = ?', note.subject_type == other.to_s)},
       {:foreign_key => :subject_id})
   end
 end

->(note) {(note.subject_type == "Volunteer") ? where('1 = 1') : none}

belongs_to :volunteer, :foreign_key => :subject_id,
  :conditions => Proc.new {['1 = ?', (subject_type == 'Volunteer')]}

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