How to get all the descendants of a given instance of a model in google app engine?

Using parent child relationship where a parent can have children while each child has only one parent, does using Children.all().ancestor(parent.key) a good solution where a child is constructed by setting parent=parent.key in the 开发者_StackOverflow社区constructor? Is the 1000 limit applies with this kind of query?

The query returns what you'd expect, all Children which have the specified parent anywhere in their ancestry. The query expresses exactly that, so I doubt there's a simpler way of doing the same thing. But App Engine does keep adding features and surprising me :-)

Possibly you need parent.key(), I think it depends whether you're in Python or Java.

Btw, it's not recommended to use ancestor-parent-child to model relationships in your data. Entity groups exist to enable transactions, not for use as a "free" ReferenceProperty. parent-child should be a low-level implementation detail, meaning either "these two entities may need to be modified in a single transaction", or perhaps "I am playing an optimization trick which allows me to use list properties without having to load the list into memory when I get the entity". As a rule of thumb, if the entities don't all "belong" to the same user, then they shouldn't be parent-child related, because relating them in that way introduces contention when different users try modify them via different datastore nodes:

http://code.google.com/appengine/docs/python/datastore/keysandentitygroups.html#Entity_Groups_Ancestors_and_Paths

Another way to get descendants (children) of a parent entity in Google App Engine that I just discovered:

childrenEntities = db.query_descendants(parentEntity).fetch(1000)

Not sure if it will be helpful to you. It was helpful to me because I was having difficulty figuring out how to access the child class(es) which were created with a python module I installed.

As others have indicated elsewhere, the 1000 limit for everything was removed in February 2010. See the linked blog entry for more details re: this.