Google App Engine查询（不过滤）实体的子项 [英] Google App Engine Query (not filter) for children of an entity

问题描述

鉴于：

  class Factory（db.Model）：
Parent-kind
 name = db.StringProperty（）
 
 class Product（db。模型）：
子类，使用Product（parent = factory）来创建
 @property 
 def factory（self）：
 return self.parent ）
 serial = db.IntegerProperty（）
  

假设有500家工厂生产了500种产品共有250,000种产品。有没有办法形成一个资源高效的查询，将返回一个特定工厂制造的500个产品？祖先方法是一种过滤器，所以使用例如Product.all（）。祖先（factory_1）需要重复调​​用数据存储。

解决方案

尽管祖先被描述为过滤器，它实际上只是更新查询以添加祖先条件。在你迭代查询之前，你不会发送请求到数据存储区，所以你有什么可以正常工作。

尽管有一个小问题：500个实体相同父母可能会伤害可伸缩性，因为写入被序列化为实体组的成员。如果您只是想跟踪生成产品的工厂，请使用ReferenceProperty：

  class Product（db.Model）：
 factory = db.ReferenceProperty（Factory，collection_name =products）
  

然后您可以获取所有产品使用：

  myFactory.products 
  

Are the children of an entity available in a Query?

Given:

class Factory(db.Model):
    """ Parent-kind """
    name = db.StringProperty()

class Product(db.Model):
    """ Child kind, use Product(parent=factory) to make """
    @property
    def factory(self):
        return self.parent()
    serial = db.IntegerProperty()

Assume 500 factories have made 500 products for a total of 250,000 products. Is there a way to form a resource-efficient query that will return just the 500 products made by one particular factory? The ancestor method is a filter, so using e.g. Product.all().ancestor(factory_1) would require repeated calls to the datastore.

解决方案

Although ancestor is described as a "filter", it actually just updates the query to add the ancestor condition. You don't send a request to the datastore until you iterate over the query, so what you have will work fine.

One minor point though: 500 entities with the same parent can hurt scalability, since writes are serialized to members of an entity group. If you just want to track the factory that made a product, use a ReferenceProperty:

class Product(db.Model):
   factory = db.ReferenceProperty(Factory, collection_name="products")

You can then get all the products by using:

myFactory.products

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