在对象化中使用'IN'查询

Question

How do I use 'IN' query in objectify? I have a 'Paper' entity and a different entity 'Schedule'. In 'Schedule' I have key of paper. Now I get a few keys of 'Paper' using some criteria. Now I want to filter those with 'scheduledDate'. I want to query 'Schedule' with something like this: get 'schedule' from 'Schedule' where 'paper key' in (List of paper keys) and 'scheduledDate' = 'some date' . How do I do this in objectify? Thanks

Answer 1

Objectify is a thin wrapper around low-level Datastore API, so IN operator behaves the same as the low-level API:

ofy.query(Schedule.class).filter("paper IN", listOfPaperKeys).filter("scheduledDate = ", someDate)

This assumes that your Schedule class has a field List<Key> paper that contains list of keys pointing to Paper entities (you could also have List<Key<Paper>> paper if you use objectify's type-safe Key s).

Note how IN performs a series of queries under the hood and combines result. So in a sense it behaves as a series of "=" operators whose result is merged:

The IN operator also performs multiple queries, one for each item in the 
specified list, with all other filters the same and the IN filter replaced with
an EQUAL filter. The results are merged, in the order of the items in the list. 
If a query has more than one IN filter, it is performed as multiple queries, 
one for each possible combination of values in the IN lists.