Merge branch 'to-one-detached' into dev

Author: greenrobot-team

objectbox-java/src/main/java/io/objectbox/relation/ToOne.java

@@ -112,16 +112,17 @@ private void ensureBoxes(TARGET target) {
             Field boxStoreField = ReflectionCache.getInstance().getField(entity.getClass(), "__boxStore");
             try {
                 boxStore = (BoxStore) boxStoreField.get(entity);
-                debugRelations = boxStore.isDebugRelations();
                 if (boxStore == null) {
                     if (target != null) {
                         boxStoreField = ReflectionCache.getInstance().getField(target.getClass(), "__boxStore");
                         boxStore = (BoxStore) boxStoreField.get(target);
                     }
                     if (boxStore == null) {
-                        throw new DbDetachedException("Cannot resolve relation for detached entities");
+                        throw new DbDetachedException("Cannot resolve relation for detached entities, " +
+                                "call box.attach(entity) beforehand.");
                     }
                 }
+                debugRelations = boxStore.isDebugRelations();
             } catch (IllegalAccessException e) {
                 throw new RuntimeException(e);
             }