User to User n:n relationship based on User role

Hello Anh,

So I'm trying to create a relationship between users as mentioned on this MB relationship plugin "Supports creating reciprocal relationships (posts-posts, users-users, …)." based on their roles.

let's say there is a "student" and "teacher". I want to create a reciprocal relationship between them, saying "student" can choose "teacher" and vice-versa. with a single field.

For now, I tried this code.

add_action( 'mb_relationships_init', function () {
    MB_Relationships_API::register( array(
        'id'   => 'stud_to_teach',
        'from' => array(
            'object_type' => 'user',
            'role__in' => 'student',
            'meta_box'    => array(
                'title'       => 'Manages',
                'field_title' => 'Select Users',
            ),
        ),
        'to' => array(
            'object_type' => 'user',
            'user_role' => 'teacher',
            'meta_box'    => array(
                'title'       => 'Manages users',
                'field_title' => 'Select Users',
            ),
        ),
    ) );
} );

what it's doing is creating 2 fields. and allowing me to add more, but it's showing every type of users. am I using the key user_role wrong? what should be the correct key?

also I'm struggling with the logic of

if logged_in_user is student then show only teacher. And if logged_in_user is teacher then show only student.