AppEngine：查询具有<缺失>记录的数据存储值 [英] AppEngine: Query datastore for records with <missing> value

问题描述

我在Google App Engine Datastore中为我的db模型创建了一个新属性。

旧：

  class Logo（db.Model）：
 name = db.StringProperty（）
 image = db.BlobProperty（）
  
  
 
 
模型）：
 name = db.StringProperty（）
 image = db.BlobProperty（）
 is_approved = db.BooleanProperty（default = False）
  

如何查询未设置'is_approved'值的Logo记录？
我试过

  logos.filter（is_approved =，无）
  

但它不起作用。
在数据查看器中，新字段值显示为。

解决方案

根据查询和索引，实体之间有一个区别， em> no 值，以及那些具有 null 值的值;和没有筛选属性的实体永远不会被查询返回。因此，无法为这些旧记录编写查询。

有用的文章是更新你的模型的模式，它表示目前唯一支持的方式来查找缺少某些属性的实体，以便检查所有这些属性。这篇文章有示例代码，展示了如何循环一大组实体并更新它们。

I created a new property for my db model in the Google App Engine Datastore.

Old:

class Logo(db.Model):
  name = db.StringProperty()
  image = db.BlobProperty()

New:

class Logo(db.Model):
  name = db.StringProperty()
  image = db.BlobProperty()
  is_approved = db.BooleanProperty(default=False)

How to query for the Logo records, which to not have the 'is_approved' value set? I tried

logos.filter("is_approved = ", None)

but it didn't work. In the Data Viewer the new field values are displayed as .

解决方案

According to the App Engine documentation on Queries and Indexes, there is a distinction between entities that have no value for a property, and those that have a null value for it; and "Entities Without a Filtered Property Are Never Returned by a Query." So it is not possible to write a query for these old records.

A useful article is Updating Your Model's Schema, which says that the only currently-supported way to find entities missing some property is to examine all of them. The article has example code showing how to cycle through a large set of entities and update them.

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