So the structure I have is this:

Record of Custom Object Type A has a related list of multiple records of Custom Object Type B.


And this is what I'm looking to do:

Object Type B has a Currency field, and Object Type A has a field that is inteded to be the Sum of that Currency field for all records in the related list. Whenever any record of Object Type B is updated, I need the Sum field on Object Type A to calculate the new sum. (Essentially it is a rollup sum, but I cannot use a rollup field because these do not have a Master-Detail relationship.)


Example:

Initial State:

• Object Type A Record SumField = 100
  • Related ObjectB Record 1 CurrencyField = 50
  • Related ObjectB Record 2 CurrencyField = 30
  • Related ObjectB Record 3 CurrencyField = 20

User Action:

• Related ObjectB Record 2 CurrencyField is changed from 30 to 80

Desired End State:

• Object Type A Record SumField = 150
  • Related ObjectB Record 1 CurrencyField = 50
  • Related ObjectB Record 2 CurrencyField = 80
  • Related ObjectB Record 3 CurrencyField = 20