Queries: order by property of related entity

is possible to sort parent items by child property ?

for example :

Author

• name
• birthday

Birthday

• date

i want list all authors, but when birthday != null must be sorting first

[greenrobot]

@smith6k If you are asking about the equivalent of a JOIN: yes this is planned soon-ish.

[RobbWatershed]

For those who are interested, here’s the workaround I finally came up with. It doesn’t implement the ability of LiveData to update datasets as they change, but it works correctly with PagedList, which is an okay tradeoff in my very own case.

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Goal

Produce a LiveData<PagedList<Content>> ordered by GroupItem.order, where Content has a N…1 relationship with GroupItem.

In ObjectBox terms, the data model is as follows :

• GroupItem entity has public ToOne<Content> content;
• Content entity has @Backlink(to = "content") public ToMany<GroupItem> groupItems;

Vanilla way of implementing (does not work when ordering with GroupItem.order for the reasons stated in this issue)

1. Produce the adequate Query<Content>
2. Get what you need by calling LivePagedListBuilder<>(new ObjectBoxDataSource.Factory<>(Query<Content>), cfg).build()

Workaround

1. Produce a Query<GroupItem>
   • use query.order(GroupItem_.order)
   • use query.link(GroupItem_.content) to apply whatever filter should be applied to Content
2. Fetch all Content ID’s with Stream.of(query.build().find()).map(gi -> gi.content.getTargetId()).toList()
3. Use an ObjectBoxPredeterminedDataSource (custom class; see code below) that will produce your LiveData<PagedList<Content>>.

This DataSource is fed with :

• the pre-determined list of ID’s produced in step 2
• a fetcher function Function<List<Long>, List<I>> that will be used to fetch your objects in the DB. In my case, the fetcher is a good old store.boxFor(Content.class).get(idList)

My implementation of ObjectBoxPredeterminedDataSource is inspired by ObjectBoxDataSource : https://gist.github.com/RobbWatershed/d6360f797d33e63606f2902b7621bf6c