Recursive delete in google app engine

Question

I'm using google app engine with django 1.0.2 (and the django-helper) and wonder how people go about doing recursive delete. Suppose you have a model that's something like this:

class Top(BaseModel):
    pass

class Bottom(BaseModel):
    daddy = db.ReferenceProperty(Top)

Now, when I delete an object of type 'Top', I want all the associated 'Bottom' objects to be deleted as well.

As things are now, when I delete a 'Top' object, the 'Bottom' objects stay and then I get data that doesn't belong anywhere. When accessing the datastore in a view, I end up with:

Caught an exception while rendering: ReferenceProperty failed to be resolved.

I could of course find all objects and delete them, but since my real model is at least 5 levels deep, I'm hoping there's a way to make sure this can be done automatically.

I've found this article about how it works with Java and that seems to be pretty much what I want as well.

Anyone know how I could get that behavior in django as well?

Answer 1

You need to implement this manually, by looking up affected records and deleting them at the same time as you delete the parent record. You can simplify this, if you wish, by overriding the .delete() method on your parent class to automatically delete all related records.

For performance reasons, you almost certainly want to use key-only queries (allowing you to get the keys of entities to be deleted without having to fetch and decode the actual entities), and batch deletes. For example:

db.delete(Bottom.all(keys_only=True).filter("daddy =", top).fetch(1000))