Objectify - 我们可以为 QueryKeys 设置 startAt(cursor) 吗？ 或者如何迭代许多 Keys 查询？

Question

I'm trying to iterate over 1000 keys of an entity at a time using this code but unlike Query , QueryKeys does not have a startAt() method for me to set it's cursor:

QueryKeys<Car> queryKeys = ofy().load().type(Car.class)
    .limit(1000)
     .keys();

// queryKeys has no startAt()
queryKeys = queryKeys.startAt(cursor)

Is there a way to loop through keys with QueryKeys just like Query ?

The reason I want to loop through keys is that I need to delete those entities from the Datastore. I might be deleting 100k - 1 million entities of a single entity type and wanted to do it in chunks because I am afraid that loading that many keys may slow things down too much or maybe error out somehow.

Answer 1

You'll want to set the cursor before calling keys, eg objectify.get().load().type(Car.class).setStartCursor(...).limit(1000).keys() .

Also, you are best to use the cursor from the previous query instead of relying on starting the query again to avoid skipping over tombstones as noted at https://cloud.google.com/datastore/docs/best-practices#deletions .

Answer 2

If you delete a page synchronously, there's not need for pagination:

 public void deleteAllEntities() {
    boolean continueDeletionLoop = true;

    while (continueDeletionLoop) {
        QueryKeys<Car> queryKeys = objectify.get().load().type(Car.class)
            .limit(1000)
            .keys();

        if (queryKeys.list().isEmpty()) {
            continueDeletionLoop = false;
        } else {
            objectify.get().delete().keys(queryKeys).now();
        }
    }

}