http://aaronvb.com/articles/44-a-polymorphic-join-table

checkpoint.rb

class Checkpoint < ActiveRecord::Base
  has_many :note_joins, as: :notable
  has_many :notes, through: :note_joins
end

create_note_joins.rb

class CreateNoteJoins < ActiveRecord::Migration
  def change
    create_table :note_joins do |t|
      t.integer :note_id
      t.integer :notable_id
      t.string  :notable_type
      t.timestamps
    end
    add_index :note_joins, [:notable_id, :notable_type]
  end
end

create_notes.rb

class CreateNotes < ActiveRecord::Migration
  def change
    create_table :notes do |t|
      t.text :content
      t.integer :user_id
      t.timestamps
    end
  end
end

location.rb

class Location < ActiveRecord::Base
  has_many :note_joins, as: :notable
  has_many :notes, through: :note_joins
end

note.rb

class Note < ActiveRecord::Base
  attr_accessible :content, :user_id
  belongs_to :notable, polymorphic: true
  belongs_to :user

  has_many :note_joins
end

note_join.rb

class NoteJoin < ActiveRecord::Base
  belongs_to :notable, polymorphic: true
  belongs_to :note
end

revised_note.rb

class Note < ActiveRecord::Base
  attr_accessible :content, :user_id
  belongs_to :notable, polymorphic: true
  belongs_to :user

  has_many :note_joins

  has_many :locations, through: :note_joins, source: :notable, source_type: 'Location'
  has_many :checkpoints, through: :note_joins, source: :notable, source_type: 'Checkpoint'
end