I have a subform in my Deals module, which is a lookup to a custom module (an offerings catalog) so I can associate multiple offerings to a single deal.  How can I have the subform filter the lookup options?  My use case is that we have a checkbox field in the custom module to determine if the offering is active or not.  I only want active offerings to show up as options in the lookup on the subform.  Users can open the lookup field to filter, but by default the active/not active filter needs to be applied.