Trying this...

src/utils/invoice.ts

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+import { db } from "../db";
+import { invoices, invoiceItems } from "../db/schema";
+import { eq } from "drizzle-orm";
+
+/**
+ * Recalculates the subtotal, tax amount, and total for a specific invoice
+ * based on its items and tax rate.
+ */
+export async function recalculateInvoiceTotals(invoiceId: string) {
+  // Fetch invoice to get tax rate
+  const invoice = await db
+    .select()
+    .from(invoices)
+    .where(eq(invoices.id, invoiceId))
+    .get();
+
+  if (!invoice) return;
+
+  // Fetch all items
+  const items = await db
+    .select()
+    .from(invoiceItems)
+    .where(eq(invoiceItems.invoiceId, invoiceId))
+    .all();
+
+  // Calculate totals
+  // Note: amounts are in cents
+  const subtotal = items.reduce((acc, item) => acc + item.amount, 0);
+
+  const taxRate = invoice.taxRate || 0;
+  const taxAmount = Math.round(subtotal * (taxRate / 100));
+
+  const total = subtotal + taxAmount;
+
+  // Update invoice
+  await db
+    .update(invoices)
+    .set({
+      subtotal,
+      taxAmount,
+      total,
+    })
+    .where(eq(invoices.id, invoiceId));
+}