Query with linkMany does not provide value if there are no entities in ToMany relation

[orestesgaolin]

Basic info (please complete the following information):

• ObjectBox version: 1.4.1
• Flutter/Dart SDK: 2.10.3
• Null-safety enabled: yes
• Reproducibility: yes, sample available here
• OS: Android, iOS, macOS
• Device/Emulator: n/a

Steps to reproduce

Example available here

1. Add several attachments to the first note by clicking on the attachment icon, don't add them to other notes

2. Update query for a Note entity that has ToMany relation using ..linkMany(Entitiy_.relation)

  ObjectBox._create(this.store) {
    noteBox = Box<Note>(store);

    final qBuilder = noteBox.query()
      ..order(Note_.date, flags: Order.descending)
      // Comment to see it working
      ..linkMany(Note_.attachment);
    queryStream = qBuilder.watch(triggerImmediately: true);

    // Add some demo data if the box is empty.
    if (noteBox.isEmpty()) {
      _putDemoData();
    }
  }

4. Hot restart the app
5. Observe that it doesn't emit values which have no entities in the above relation

6. Comment out line 23 in objectbox.dart file, hot restart the app, and observe this entity to be shown again

Expected behavior

Query using .linkMany should emit values also when there are no entities in the ToMany relation

Code

Sample available here https://github.com/objectbox/objectbox-dart/pull/402/files

[greenrobot-team]

Thanks for the report. However, why would you expect a query with the linkMany condition to also return objects without related objects? So basically being equivalent to the query without the linkMany condition?

Note that typically the link conditions are used with a condition on the related entity, so e.g.
linkMany(Note_.attachment, Attachment_.content.contains("something")).

[orestesgaolin]

I think I see where my expectation does not meet the reality - I don't pass any condition to the linkMany because I want all the possible related objects, even when there are no objects yet. The reason I want to use linkMany is that I want to be notified about changes to the related objects as well. In the example with notes and attachments if the attachment name changes, I want this to be reflected in the main query. Without using linkMany I won't be notified about the change of the linked attachment name (correct me if there's a different way to achieve that).

The workaround I use right now uses 2 instances of the same query. One of them without linkMany, the other with linkMany. Then I merge 2 streams with Rx.merge. In pseudo code it would be something like:

    final qBuilder1 = noteBox.query()
      ..order(Note_.date, flags: Order.descending)
      ..linkMany(Note_.attachment);
    final qBuilder2 = noteBox.query()
      ..order(Note_.date, flags: Order.descending);
    return Rx.merge([qBuilder1.watch(triggerImmediately: true), qBuilder2.watch(triggerImmediately: true)])
               .distinct();

Case	Without linkMany	With linkMany
0 related objects	✅	❌
1 related object	✅ but does not update when related object changed	✅
remove last object	✅	❌ does not update the list

[greenrobot-team]

Got it. In that case I recommend to write your own watch implementation. You can look at how it is done for queries and write a custom version that listens to the entity types you are interested in (e.g. replace queriedEntities with your own set):

objectbox-dart/objectbox/lib/src/native/query/builder.dart

Lines 33 to 53 in d72f51e

	Stream<Query<T>> watch({bool triggerImmediately = false}) {
	final queriedEntities = HashSet<Type>();
	_fillQueriedEntities(queriedEntities);
	final query = build();
	late StreamSubscription<void> subscription;
	late StreamController<Query<T>> controller;
	final subscribe = () {
	subscription = _store.entityChanges.listen((List<Type> entityTypes) {
	if (entityTypes.any(queriedEntities.contains)) {
	controller.add(query);
	}
	});
	};
	controller = StreamController<Query<T>>(
	onListen: subscribe,
	onResume: subscribe,
	onPause: () => subscription.pause(),
	onCancel: () => subscription.cancel());
	if (triggerImmediately) controller.add(query);
	return controller.stream;
	}

Then you can use any query you want with it.

[github-actions]

Without additional information, we are unfortunately not sure how to resolve this issue. Therefore this issue has been automatically closed. Feel free to comment with additional details and we can re-open this issue.