Programming Google App Engine with Java: Build & Run Scalable Java Applications on Google's Infrastructure by Dan Sanderson

Author:Dan Sanderson [Sanderson, Dan]
Language: eng
Format: azw3
Publisher: O'Reilly Media
Published: 2015-06-30T04:00:00+00:00

SELECT * FROM Message WHERE ANCESTOR IS KEY(MessageBoard, 'The_Archonville_Times') ORDER BY post_date DESC LIMIT 10

Most queries that use an ancestor filter need custom indexes. There is one unusual exception: a query does not need a custom index if the query also contains equality filters on properties (and no inequality filters or sort orders). In this exceptional case, the “merge join” algorithm can use a built-in index of keys along with the built-in property indexes. In cases where the query would need a custom index anyway, the query can match the ancestor to the keys in the custom index.

In Java, you create an ancestor query by passing the ancestor key to the Query constructor:

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