Update Opportunity Multiselect Field Based On Field In Related Revenue Line Items

I have a custom multi-select field in an Opportunity (call it "system-lines") that I want to keep updated based on the value of a field in all related RLIs (call it "system"). The Opportunity field value is determined by a Business Rule that maps the RLI Field Value to an appropriate value in the Opportunity field dropdown list. 

When an RLI is created or the RLI:system is changed, I want to use the Business Rule to get the system to system-line mapped value, then apply that value to the Opportunity:system-lines multi-select field. 

One issue I'm having is that I believe I need to cycle through all Opportunity-related RLIs to rebuild the Opportunity:system-lines field due to not being able to judge the impact of any one RLI:system change to the overall Opportunity:system-lines field. (if that makes sense). 

Issue:

  • Looping through all RLIs for an Opportunity.
  • Setting a multi-select field with  a Business Rule evaluation result. 

Thanks for guidance on any part of this. I'm just learning SugarBPM, so any help is appreciated.