cloud_firestore Query compound query with order by

[libterty]

I'm new to firebase store, i'm don't really understand why compound query with orderby, and orderby has to be on the same property. And i take a look at source code, not sure if it's here thrown the exception.

 if (_isInequality(operator)) {
    if (hasInequality == null) {
      hasInequality = field;
    } else {
      assert(
        hasInequality == field,
        'All where filters with an inequality (<, <=, >, or >=) must be '
        "on the same field. But you have inequality filters on '$hasInequality' and '$field'.",
      );
    }
  }

And the sample query is

Query query = FirebaseFirestore.instance
    .collection('media')
    .where('type', isNotEqualTo: 'file')
    .orderBy('type', descending: true) // if this line don't add it will throw error
    .orderBy('timestamp', descending: true)
    .limit(50);