How to insert multiple rows from a single query using eloquent/fluent

I have the following query:

  $query = UserSubject::where('user_id', Auth::id())->select('subject_id')->get();

and as expected I get the following result:

[{"user_id":8,"subject_id":9},{"user_id":8,"subject_id":2}]

Is there a way of copying the above result into another table so that my table looks like this?

ID|user_id|subject_id
1 |8      |9
2 |8      |2

The problem I have is that the $query can expect any number of rows and so im unsure how to iterate through an unknown number of rows.