Find id by username

[CromeX]

Hi,
I am new to userspice and my experience with PHP is two months, so forgive me if I am a bit slow. I have added a new column to the users table. I want to add a random number to this column for each user after register, trying to do that in join.php.
Here is what I have done so far

this is to get the id
<pre>

Code:

$user_name=Input::get('username');
$user_n = $db->get('users',['username','=',$user_name]);

</pre>


then insert it

Code:

$db->update('deposit_id',$the_id,$fields);

nothing is happening, what am I doing wrong?
Any help would be appreciated trying to recommend this app for a project.
Thanks in advance.