如何在 GAE Python NDB 中获取最新数据 [英] How to fetch the latest data in GAE Python NDB

问题描述

我正在使用 GAE Python.我有两个根实体:

I am using GAE Python. I have two root entities:

class X(ndb.Model):  
    subject = ndb.StringProperty()  
    grade = ndb.StringProperty()  

class Y(ndb.Model):  
    identifier = ndb.StringProperty()  
    name = ndb.StringProperty()  
    school = ndb.StringProperty()  
    year = ndb.StringProperty()  
    result = ndb.StructuredProperty(X, repeated=True)  

由于谷歌将我们的数据存储在多个数据中心，当我们进行如下查询时，我们可能无法获得最新的数据(以防某些更改已被放置"):

Since google stores our data across several data centers, we might not get the most recent data when we do a query as shown below(in case some changes have been "put"):

def post(self):  
    identifier = self.request.get('identifier')  
    name = self.request.get('name')  
    school = self.request.get('school')  
    year = self.request.get('year')  
    qry = Y.query(ndb.AND(Y.name==name, Y.school==school, Y.year==year))  
    record_list = qry.fetch()  

我的问题:我应该如何修改上面的fetch操作以始终获取最新的数据

My question: How should I modify the above fetch operation to always get the latest data

我已经浏览了相关的 google 帮助文档，但无法了解如何在此处应用

I have gone through the related google help doc but could not understand how to apply that here

根据艾萨克回答的提示，以下是解决方案吗(latest_record_data"是否包含实体的最新数据):

Based on hints from Isaac answer, Would the following be the solution(would "latest_record_data" contain the latest data of the entity):

def post(self):  
    identifier = self.request.get('identifier')  
    name = self.request.get('name')  
    school = self.request.get('school')  
    year = self.request.get('year')  
    qry = Y.query(ndb.AND(Y.name==name, Y.school==school, Y.year==year))  
    record_list = qry.fetch()  
    record = record_list[0]  
    latest_record_data = record.key.get()  

推荐答案

在应用引擎上有几种方法可以获得强一致性，最常见的是使用获取而不是查询和使用祖先查询.

There's a couple ways on app engine to get strong consistency, most commonly using gets instead of queries and using ancestor queries.

要在您的示例中使用 get，您可以将名称编码到实体键中:

To use a get in your example, you could encode the name into the entity key:

class Y(ndb.Model):
  result = ndb.StructuredProperty(X, repeated=True)

def put(name, result):
  Y(key=ndb.Key(Y, name), result).put()

def get_records(name):
  record_list = ndb.Key(Y, name).get()
  return record_list

祖先查询使用相似的概念来做一些更强大的事情.例如，获取具有特定名称的最新记录:

An ancestor query uses similar concepts to do something more powerful. For example, fetching the latest record with a specific name:

import time

class Y(ndb.Model):
  result = ndb.StructuredProperty(X, repeated=True)

  @classmethod
  def put_result(cls, name, result):
    # Don't use integers for last field in key. (one weird trick)
    key = ndb.Key('name', name, cls, str(int(time.time())))
    cls(key=key, result=result).put()

  @classmethod
  def get_latest_result(cls, name):
    qry = cls.query(ancestor=ndb.Key('name', name)).order(-cls.key)
    latest = qry.fetch(1)
    if latest:
      return latest[0]

祖先"是实体的第一对键.只要您可以将至少具有第一对的密钥放入查询中，您就会获得强一致性.

The "ancestor" is the first pair of the entity's key. As long as you can put a key with at least the first pair into the query, you'll get strong consistency.

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