[rules-users] Verify if an objects attribute exists

[lnguyen]

I'm having difficulty writing this rule.

Background information:
A User can have a supervisor but it's not required.

When viewing User information I want to verify that the logged in user is
either the current user or the currentUser's supervisor but I throw an
exception when the supervisor object is null on the currentUser.

The target is the User being viewed and I have a global object that is the
current user. How can I check to see if the supervisor exists before
evaluating if the currentUser and the supervisor are equal?

rule 'CanViewUserData'
dialect 'mvel'
when
    $user : User()
    $supervisor : User( ) from $user.userProfile.supervisor
    $check: PermissionCheck(target == $user, action == "viewUserData")
    eval(isUserMatching($user)) || eval (isUserMatching($supervisor))
then
    System.out.println("The currentUser CanViewUserData");
    $check.grant();
end

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