address.rb

class Address < ActiveRecord::Base
  has_many :phonings, :as => :phonable
  has_many :phone_numbers, :through => :phonings

  accepts_nested_attributes_for :phonings, :allow_destroy => true

end

address_test.rb

  test "By default adding the same phone number will create a new phone number row when adding a second phone number" do
    a = Address.new(default_address(
                                    "phonings_attributes"=>
                                    {"1289233155995"=>
                                      {"phone_number_attributes"=>
                                        {"phone_number"=>"17032221234"},
                                        "_destroy"=>"false"}
                                    }
                                    ))
    a.save!
    assert_equal "Cville", Address.find(a.id).city
    assert_equal "17032221234", Address.find(a.id).phone_numbers.first.phone_number
    b = Address.new(default_address(:address1 => "234 Test",
                                    "phonings_attributes"=>
                                    {"1289233155995"=>
                                      {"phone_number_attributes"=>
                                        {"phone_number"=>"17032221234"},
                                        "_destroy"=>"false"}
                                    }
                                    ))
    b.save!
    assert_equal "Cville", Address.find(b.id).city
    assert_equal "17032221234", Address.find(b.id).phone_numbers.first.phone_number
    assert_equal @original_phone_number_count + 2, PhoneNumber.count
    assert_equal 2, PhoneNumber.where(:phone_number => "17032221234").count
  end

address_test_2.rb

  test "When you override the autosave associated for the join table then you can point multiple addresses at the same phone number object." do

    # overriding autosave_associated_records_for_phone_numbers
    Phoning.send :define_method, :autosave_associated_records_for_phone_number do
      if new_phone_number = PhoneNumber.where(:phone_number =>  phone_number.phone_number).first then
           self.phone_number = new_phone_number
      else
        self.phone_number.save!
        self.phone_number_id = self.phone_number.id
      end
    end

    a = Address.new(default_address(
                                    "phonings_attributes"=>
                                    {"1289233155995"=>
                                      {"phone_number_attributes"=>
                                        {"phone_number"=>"17032221234"},
                                        "_destroy"=>"false"}
                                    }
                                    ))
    a.save!
    assert_equal "Cville", Address.find(a.id).city
    assert_equal "17032221234", Address.find(a.id).phone_numbers.first.phone_number
    b = Address.new(default_address(:address1 => "234 Test",
                                    "phonings_attributes"=>
                                    {"1289233155995"=>
                                      {"phone_number_attributes"=>
                                        {"phone_number"=>"17032221234"},
                                        "_destroy"=>"false"}
                                    }
                                    ))
    b.save!
    assert_equal "Cville", Address.find(b.id).city
    assert_equal "17032221234", Address.find(b.id).phone_numbers.first.phone_number
    assert_equal 3, PhoneNumber.count
    assert_equal 1, PhoneNumber.where(:phone_number => "17032221234").count
  end

person.rb

class Person < ActiveRecord::Base
  has_many :phonings, :as => :phonable
  has_many :phone_numbers, :through => :phonings
end

phone_number.rb

class PhoneNumber < ActiveRecord::Base
  has_many :phonings, :dependent => :destroy
  has_many :addresses, :through => :phonings, :source => :phonable, :source_type => "Address"
  has_many :people, :through => :phonings, :source => :phonable, :source_type => "Person"
end

phoning.rb

class Phoning < ActiveRecord::Base
  belongs_to :phone_number, :autosave => true
  belongs_to :phonable, :polymorphic => true

  accepts_nested_attributes_for :phone_number

  def autosave_associated_records_for_phone_number
    if new_phone_number = PhoneNumber.where(:phone_number => phone_number.phone_number).first then
      self.phone_number = new_phone_number
    else
      self.phone_number.save!
      self.phone_number_id = self.phone_number.id
    end
  end
end

phoning_original.rb

class Phoning < ActiveRecord::Base
  belongs_to :phone_number, :autosave => true
  belongs_to :phonable, :polymorphic => true

  accepts_nested_attributes_for :phone_number
end