Chaining association queries on instance unexpectedly resets foreign keys

gistfile1.rb

require 'test/unit/assertions'
require 'dm-core'
require 'dm-sqlite-adapter'
require 'dm-migrations'

DataMapper.setup :default, "sqlite::memory:"

class Activity
  include DataMapper::Resource
  property :id, Serial

  belongs_to :post, required: false
  belongs_to :user, required: false
end

class Post
  include DataMapper::Resource
  property :id, Serial

  belongs_to :user

  has n, :activities
end

class User
  include DataMapper::Resource
  property :id, Serial

  has n, :posts
  has n, :activities

  def all_activities
    activities | posts.activities
  end
end

DataMapper.auto_migrate!

user = User.create
user.activities.create

post = Post.create(user: user)
post.activities.create

user_activity, post_activity = user.all_activities

include Test::Unit::Assertions

assert_equal user.id, user_activity.user_id
assert_equal nil, user_activity.post_id

assert_equal post.id, post_activity.post_id
assert_equal nil, post_activity.user_id
# FAIL:
#   <nil> expected but was <1>
#   post_activity.dirty_attributes # => {:user_id => 1}