Reference property for parent/ancestor relation?

What is the best practice when it come开发者_Python百科s to reference properties for parent/ancestor relations in AppEngine? Should I add a reference property pointing from the parent to the child to make it easy to access the child in the parent, or should I just "suck it up" and do a ancestor query from the parent to get it's children.

If there is only one child, you could use a known key_name so you can directly fetch the child (since you know the parent's key). This can be useful because any time you know the parent's key name or id you can easily fetch the child directly.

child_key = db.Key.from_path('ChildKind', 'knownname', parent=parent_key)
child = db.get(child_key)

If there are multiple children you could potentially store a list of keys (it can be unindexed) on the parent, or use an ancestor query. I would say it depends how your data is updated, reported on (presented), and the volume of data. In other words it depends on the needs of your application.