DataMapper: an opportunity for optimization: don't need to retrieve existing join resources when replacing them

test.rb

require "rubygems"
require "datamapper"

DataMapper::Logger.new($stdout, :default)
DataMapper.setup(:default, "sqlite::memory:")

class Source
  include DataMapper::Resource

  property :id, Serial

  has n, :joins
  has n, :targets, :through => :joins
end

class Join
  include DataMapper::Resource

  belongs_to :source, :key => true
  belongs_to :target, :key => true

end

class Target
  include DataMapper::Resource

  property :id, Serial

  has n, :joins
  has n, :sources, :through => :joins
end

DataMapper.auto_migrate!

s = Source.create

10.times { Target.create }

s.targets = Target.all
s.save

s.targets = [Target.first]
s.save