support to-one relation cycles (#264)

Author: vaind

objectbox/lib/src/native/box.dart

@@ -166,7 +166,15 @@ class Box<T> {
         throw StateError(
             'Invalid state: can only use _put() on an entity with relations when executing from inside a write transaction.');
       }
-      if (_hasToOneRelations) _putToOneRelFields(object, mode, tx);
+      if (_hasToOneRelations) {
+        // In this case, there may be relation cycles so get the ID first.
+        if ((_entity.getId(object) ?? 0) == 0) {
+          final newId = C.box_id_for_put(_cBox, 0);
+          if (newId == 0) throwLatestNativeError(context: 'id-for-put failed');
+          _entity.setId(object, newId);
+        }
+        _putToOneRelFields(object, mode, tx);
+      }
     }
     _builder.fbb.reset();
     var id = _entity.objectToFB(object, _builder.fbb);

objectbox/test/relations_test.dart

@@ -464,6 +464,18 @@ void main() {
     final read = box.get(1)!;
     root.expectSameAs(read);
   });
+
+  test('cycles', () {
+    final a = RelatedEntityA();
+    final b = RelatedEntityB();
+    a.relB.target = b;
+    b.relA.target = a;
+    env.store.box<RelatedEntityA>().put(a);
+
+    final readB = env.store.box<RelatedEntityB>().get(b.id!)!;
+    expect(a.relB.targetId, readB.id!);
+    expect(readB.relA.target!.id, a.id);
+  });
 }
 
 int toInt(dynamic e) => e.tInt as int;