[SOLVED] Lookup filtered

Hi PHPR community,

I want to know how can you filter a lookup on a detail table based on a column value on a Master table?


Example:
MAINFORM - Master table
id
system_id


FORMDETAIL - Detail table
id
mainform_id
category_id (lookup should only show depending on value of system_id on MAINFORM master table)


SYSTEM
id
name


CATEGORY
id
name
system_id




Is it only possible via event custom code?