Laravel 5.8: Uniqueness validation multiple columns with nested array wildcard (*.)

How to validate the uniqueness of combined column value in the nested array?

The input data is

$request = [
    [id, vol, number], 
    [id2, vol2, number2], 
    ...
];

The combination of vol and number must be unique with the data in magazines table.

id | vol | number
------------------
0  | 001 | 0001
1  | 001 | 0002
2  | 002 | 0001

Here's what my code looks like for now.

$validatedRequest = $request->validate([
            '*.id' => ['required', 'string'],
            '*.vol' => ['required', 'string'],
            '*.number' => ['required', 'string'], //need to add some rule here, I guess
        ]);

I've read the docs about how to validate nested array data with Rule::forEach() and also read the tutorial on how to validate the uniqueness on multiple column. However, I still cannot figure the way to access the *.vol data when creating rule for number.

via Chebli Mohamed