从’django-mptt`使用`get_ancestors`函数时出现错误的结果

我正在开发一个使用django-mptt的项目,但是当我使用get_ancestors函数时,我得到了奇怪的结果.这是一个例子.
我有一个创建了一个简单的模型,继承自MPTTModel：

class Classifier(MPTTModel):
    title = models.CharField(max_length=255)
    parent = TreeForeignKey('self',null = True,blank = True,related_name = 'children')

    def __unicode__(self):
        return self.title

以下是适用于此模型的功能：

def test_mptt(self):
    # Erase all data from table
    Classifier.objects.all().delete()

    # Create a tree root
    root,created = Classifier.objects.get_or_create(title=u'root',parent=None)

    # Create 'a' and 'b' nodes whose parent is 'root'
    a = Classifier(title = "a")
    a.insert_at(root,save = True)
    b = Classifier(title = "b")
    b.insert_at(root,save = True)

    # Create 'aa' and 'bb' nodes whose parents are
    # 'a' and 'b' respectively
    aa = Classifier(title = "aa")
    aa.insert_at(a,save = True)
    bb = Classifier(title = "bb")
    bb.insert_at(b,save = True)

    # Create two more nodes whose parents are 'aa' and 'bb' respectively
    aaa = Classifier(title = "aaa")
    aaa.insert_at(aa,save = True)
    bba = Classifier(title = "bbb")
    bba.insert_at(bb,save = True)

    # Select from table just created nodes
    first = Classifier.objects.get(title = "aaa")
    second = Classifier.objects.get(title = "bbb")

    # Print lists of selected nodes' ancestors:
    print first.get_ancestors(ascending=True,include_self=True)
    print second.get_ancestors(ascending=True,include_self=True)

我希望在输出上看到下一个值：

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