Is the parent 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.
-
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
Thomas L Holaday : The docs call it a filter, but it isn't? Mercy sakes, what next. The use case for writes is each Factory creates zero to ten Products / day with a minimum time between products of ten seconds ("bursty"). 95% of Product reads will be Factory-as-a-whole. Is parent-child or ReferenceProperty better?mcobrien : I think the recommendation is to use entity groups from transactions only, so a ReferenceProperty would be better, although there probably isn't a huge difference...Nick Johnson : There's nothing wrong with 500 elements in the same entity group - it's the update rate that counts.
0 comments:
Post a Comment