The advanced search in /hardware produces incorrect results if the serial is combined with another field like category.
There is a typo as the fieldname 'product_key' doesn't exist. Change this to 'serial'.
Also change the last If-Statement from ->orWhere() to ->where(). Now additional fields like custom fields can be combined with other searches in an And-Clause.
I think this function could be simplified further, but this is the minimal bugfix.
eced1ab77f