It does the quantity x price from the transactionline and stores that in transactionaccountingline. So for example, if you edit a transaction, the code accepting the POST has to update two records per line, not just one.

Unless of course the extension field in transactionaccountingline is seeded by a SQL Function that does that automatically. Which might be a preferred approach for your implementation. The downside of SQL functions though, they tend to be invisible to others maintaining the code.