Logic Hook not working for Relate field

Hello Experts
Thanks for helping me out. I need your help again .My Logic hook is not working accordingly.
I have dropdown on Tasks Module when i select “Services” some tabs hide and some show and if i select “Research” again same procedure. Problem is when i choose services then i have Tab "Assigned To " and it is working fine for services. But when i select “Research” then i simply hide that Assigned to tab. And every time i shows value admin in my listing.
Below is my logic hook code

<?php if(!defined('sugarEntry') || !sugarEntry) die('Not a valid Entry Point'); class Assign{ function sumit($bean , $event, $arguments) { if($bean->select_order_type_c == "Services") { $bean->sales_person_involve_c = "No"; // var_dump($bean->assigned_user_name); // exit; } else if($bean->select_order_type_c == "Research") { $bean->assigned_user_name == 'Null'; } } } ?>

I didn’t quite understand your question well. What logic hook is this? before_save ? after_ui_footer ?
Are you conditionally saving or showing the data?

Not sure on what you are trying but the code suggests instead of
$bean->assigned_user_name == 'Null';
it should be
$bean->assigned_user_id == '';