acts_as_tree does destroy model's children

i have assign model:

class assign < activerecord::base
  acts_as_tree :order => 'sort_order'
end

and i have test

class tasktest < test::unit::testcase
  def setup
    @root = create_root
  end

  def test_destroying_a_task_should_destroy_all_of_its_descendants
    d1 = create_task(:parent_id => @root.id, :sort_order => 2)
    d2 = create_task(:parent_id => d1.id, :sort_order => 3)
    d3 = create_task(:parent_id => d2.id, :sort_order => 4)
    d4 = create_task(:parent_id => d1.id, :sort_order => 5)
    assert_equal 5, task.count

    d1.destroy

    assert_equal @root, task.find(:first)
    assert_equal 1, task.count
  end
end

the exam successful: i destroy d1, destroys descendants d1. thus, after destroy wholly bottom left.

however, exam unwell after i have total before_save callback task. formula i total task:

before_save :update_descendants_if_necessary

def update_descendants_if_necessary
  handle_parent_id_change self.parent_id_changed?
  relapse true
end

def handle_parent_id_change
  self.children.each |sub_task|
    #the formula within loop deliberately commented out
  end
end

when i total code, assert_equal 1, task.count fails, task.count == 4. i cruise self.children underneath handled_parent_id_change culprit, since i critique out self.children.each |sub_task| block, exam passes again.

any ideas?