In order to retrieve all models that are related to a parent model, you can use query predicates with the query API:

```dart
Future<void> fetchAllCommentsForPostId(String postId) async {
  final comments = await Amplify.DataStore.query(
    Comment.classType,
    where: Comment.POST.eq(postId),
  );
  print('Comments: $comments');
}
```
