appengine python NDB 排序顺序无效

Question

我尝试在 model.py 中使用这些模型获取最后修改的条目：

class Example(ndb.Model):
    ....
    modified = ndb.DateTimeProperty(auto_now=True)
    created = ndb.DateTimeProperty(auto_now_add=True)

查询代码：

for each in ['Example', '...'] 
    model_class = webapp2.import_string('model.%s' % each)
    q = model_class.query()
    q.order(-model_class.modified)
    last_modified_entity = q.get()  # does not sort the entities

我也试过：

for each in ['Example', '...'] 
    model_class = webapp2.import_string('model.%s' % each)
    q = model_class.query()
    sort_prop = ndb.GenericProperty('modified')
    q.order(-sort_prop)
    last_modified_entity = q.get()  # does not sort the entities

Answer 1

在 Query 对象上调用 .order() 不会改变 Query 对象。它 returns 一个带有该订单的新 Query 对象，但不会改变原始 Query 对象。来自 NDB Queries（强调我的）：

Instead of specifying an entire query filter in a single expression, you may find it more convenient to build it up in steps: for example:

query1 = Account.query()  # Retrieve all Account entitites
query2 = query1.filter(Account.userid >= 40)  # Filter on userid >= 40
query3 = query2.filter(Account.userid < 50)  # Filter on userid < 50 too

query3 is equivalent to the query variable from the previous example. Note that query objects are immutable, so the construction of query2 does not affect query1 and the construction of query3 does not affect query1 or query2.

来自远程的示例 api shell:

>>>> from models.notification import Notification
>>>> query = Notification.query()
>>>> query
Query(kind='Notification')
>>>>
>>>> # this returns a new ordered query
>>>> query.order(Notification.created_on)
Query(kind='Notification', orders=...)
>>>> 
>>>> # but it does not alter the original query
>>>> query
Query(kind='Notification')
>>>>
>>>> # this is how you get the ordered query in a variable
>>>> ordered_query = query.order(Notification.created_on)
>>>> ordered_query
Query(kind='Notification', orders=...)
>>>> query
Query(kind='Notification')

因此请更改您的代码以改用此代码：

q = q.order(-sort_prop)