简化数据库关系设计

I have this little Laravel project where members can pay to attend to a class, pay insurance, event, or pay to pass a special type of exams.

So I have 5 tables members, classes, insurance, events and exams, and I need to add the payments table so I can track members payments for each one of those tables.

So for the moment the payments table is going to be like this :

id      date            amount      member_id   class_id    insurance_id    event_id    exam_id
1       2019-01-01      150         2           1           NULL            NULL        NULL
2       2019-01-01      250         11          NULL        14              NULL        NULL
3       2019-01-01      220         15          NULL        NULL            6           NULL
4       2019-01-01      350         32          NULL        NULL            NULL        8

Is there a better way to this without having : class_id, insurance_id, event_id, exam_id fields in the payments table, because this is going to make the relationship more complexe I guess, and to also maybe simply DB queries using Laravel as a PHP Framework.