NDB查询返回零结果。数据存储显示结果 [英] NDB query returns zero results. Datastore shows the result

问题描述

以下是我的模型：

$ b在运行查询的过程中发现了这个特殊问题，确认该记录存在，并返回零计数。
$ b

  class描述（ndb.Model）：
 description = ndb.TextProperty（）
 time_posted = ndb.DateTimeProperty（auto_now_add = True ）
 uuid = ndb.StringProperty（）
 $ b $ class Examine（ndb.Model）：
 updated = ndb.DateTimeProperty（auto_now = True，auto_now_add = True）
描述= ndb.StructuredProperty（Description，repeated = True）
 active = ndb.KeyProperty（kind =描述）
 slug = ndb.StringProperty（indexed = True）
  
 
假设我正在运行以下内容，确认数据存储区中存在特定的UUID：
 
 
  d_id ='ef531b70-3486-11e3-9500-ef31d661e6b2'
 cnt = Description.query（Description.uuid == d_id）.count（）
  
我将以cnt结果为0。有人可以向我解释为什么会发生这种情况吗？
解决方案
 Datastore查询最终一致。这意味着如果底层数据发生变化，有时一个查询将无法反映这种变化。 
 
 
 
为了解决这个问题，您可以将您的数据存储和查询构建为强一致： https://developers.google.com/appengine/docs/python/datastore/structuring_for_strong_consistency  
 
 
 
如果描述实体被保存在检查的父键中，那么下面的查询将是非常一致的： 
 
 
  cnt = Description.query（ ancestor = ExamineKey）.filter（Description.uuid == d_id）.count（）
  
 
I found this peculiar problem where running a Query, confirming the record exists, returns a count of zero.


Here are my models:
class Description(ndb.Model):
    description = ndb.TextProperty()
    time_posted = ndb.DateTimeProperty(auto_now_add=True)
    uuid = ndb.StringProperty()

class Examine(ndb.Model):
    updated = ndb.DateTimeProperty(auto_now=True, auto_now_add=True)
    descriptions = ndb.StructuredProperty(Description, repeated=True)
    active = ndb.KeyProperty(kind=Description)
    slug = ndb.StringProperty(indexed=True)
Assume that I'm running the following, confirming that the specific UUID does exist in the datastore:
d_id = 'ef531b70-3486-11e3-9500-ef31d661e6b2'
cnt = Description.query(Description.uuid == d_id).count()
I will receive 0 as a result for cnt. Could somebody explain to me why this is happening?
解决方案
Datastore queries are eventually consistent. Meaning that if the underlying data changes, sometimes a query will fail to reflect this change. 

To remedy this you can structure your datastore and queries to be strongly consistent: https://developers.google.com/appengine/docs/python/datastore/structuring_for_strong_consistency

If the description entity was saved within a parent key of examine then the following query would be strongly consistent:
cnt = Description.query(ancestor=ExamineKey).filter(Description.uuid == d_id).count()


                        
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