计算mongoengine ListField元素

Question

我很难用MySQL将我的模型从标准Django orm迁移到MySQL

我有以下模型，在旧结构中工作得很好：

class Place(Document):

    name    = StringField()
    acronym = StringField()

    parent = ReferenceField('self')
    hierarchy = ListField(ReferenceField('self'))
    hierarchy_size = IntField()

    @classmethod
    def preSave(instance, sender, **kwargs):
        instance.hierarchy_size = len(instance.hierarchy)

signals.pre_save.connect(Place.preSave, sender=Place)

但是，在使用新的mongoengine时，我面临着问题，无法像以前那样访问对象层次结构大小。接收：

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/awesome_user/workspace/awesome_project/project/models.py", line 728, in importPlacesFromMySQL
    myCountry.save()
  File "/Users/awesome_user/workspace/awesome_project/site_env/lib/python2.7/site-packages/mongoengine/document.py", line 220, in save
    signals.pre_save.send(self.__class__, document=self)
  File "/Users/awesome_user/workspace/awesome_project/site_env/lib/python2.7/site-packages/blinker/base.py", line 267, in send
    for receiver in self.receivers_for(sender)]
  File "/Users/awesome_user/workspace/awesome_project/project/models.py", line 804, in preSave
    instance.hierarchy_size = len(instance.hierarchy)
TypeError: object of type 'ListField' has no len()

有没有人知道如何访问实例的属性，比如我的例子中的length？

Answer 1

您在定义preSave类方法时出错了。您正在尝试从hierarchy ListField中从Place类(而不是实例)获取len。这是因为您使用的是方法的第一个参数-它不是一个实例，而是一个类本身。改用第三个参数：

@classmethod
def preSave(cls, sender, document, **kwargs):
    document.hierarchy_size = len(document.hierarchy)

Answer 2

len(object.field)将检索所有子文档，然后计数。如果只需要数组的长度而不检索所有文档，则可以使用len(object.to_mongo()["field"])