Getting user role and storing it in a field

Hello!
I want to create a custom system notification on creation of new task, the issue is I want this notification to only trigger if the creator of the task has a certain role.
So I modified the task doctype, created a link field (Links to User) called document_owner and set the default value to “user”.
I want to create another field which will store the user role so I can use it as a condition in my notification.
I tried document_owner.role .roles .user_roles but nothing worked.
Any help is appreciated.