Add ability to subtract allowances (#108)

Closes #104

Reviewed-on: #108

backend/db.go

@@ -562,11 +562,39 @@ func (db *Db) AddAllowanceAmount(userId int, allowanceId int, request AddAllowan
 	}
 
 	if allowanceId == 0 {
+		if remainingAmount < 0 {
+			var userBalance int
+			err = tx.Query("select balance from users where id = ?").
+				Bind(userId).ScanSingle(&userBalance)
+			if err != nil {
+				return err
+			}
+			if remainingAmount > userBalance {
+				return fmt.Errorf("cannot remove more than the current balance: %d", userBalance)
+			}
+		}
 		err = tx.Query("update users set balance = balance + ? where id = ?").
 			Bind(remainingAmount, userId).Exec()
 		if err != nil {
 			return err
 		}
+	} else if remainingAmount < 0 {
+		var progress int
+		err = tx.Query("select balance from allowances where id = ? and user_id = ?").
+			Bind(allowanceId, userId).ScanSingle(&progress)
+		if err != nil {
+			return err
+		}
+
+		if remainingAmount > progress {
+			return fmt.Errorf("cannot remove more than the current allowance balance: %d", progress)
+		}
+
+		err = tx.Query("update allowances set balance = balance + ? where id = ? and user_id = ?").
+			Bind(remainingAmount, allowanceId, userId).Exec()
+		if err != nil {
+			return err
+		}
 	} else {
 		// Fetch the target and progress of the specified allowance
 		var target, progress int