I’ve the following piece of model:

Deal -< TradeInstruction // a deal has multiple trade instructions 

and need to calculate balance for a Deal which is transient and defined as follows:

public BigDecimal sumDealTradeInstructions(Long dealId) {     return getDealTradeInstructions(dealId).stream()             .filter(ti -> ti.getStatus() != CANCELLED)             .map(TradeInstruction::getValue)             .reduce(ZERO, BigDecimal::add); } 

Now, in code there’s no relation between these two entities (yes, both beings are JPA entities) and the method for calculating the balance is defined (along with a handful of other let put this here methods) in TradeInstructionService. I don’t think this is a good place for this responsibility. In a perfect world there would be mapping between the entities and balance would be calculated in Deal:

class Deal {   List<TradeInstruction> tradeInstructions ...     BigDecimal getBalance() {      return tradeInstructions ...   } } 

but here it’s not possible to introduce the mapping, hence I want to move the calculation to DealBalanceCalculator.

Questions:

1. Is my reasoning correct?
2. How do I obtain the list of TradeInstructions in this to-be-defined calculator to calculate the balance? Via service, repository, another being?