Add realistic loan amortization with currentBalance back-calculation

- Extend schema with effectiveAnnualRate, totalInterest, totalAmount
- Back-calculate paid months from currentBalance and rebuild schedule
- Allow schedule calculation from termMonths without monthlyPayment
- Handle NaN form values gracefully
- Show effective rate, total interest and total amount in UI
- Add amortization unit tests

convex/loans.ts

@@ -5,7 +5,6 @@ import {
   buildSchedule,
   computePaymentFromTerm,
   computeTermFromPayment,
-  currentBalanceFromSchedule,
 } from "./lib/amortization";
 
 const loanValidator = v.object({
@@ -18,10 +17,13 @@ const loanValidator = v.object({
   categoryId: v.optional(v.id("categories")),
   principal: v.number(),
   annualInterestRate: v.number(),
+  effectiveAnnualRate: v.optional(v.number()),
   monthlyPayment: v.optional(v.number()),
   termMonths: v.optional(v.number()),
   startDate: v.string(),
   currentBalance: v.optional(v.number()),
+  totalInterest: v.optional(v.number()),
+  totalAmount: v.optional(v.number()),
   status: v.union(v.literal("aktiv"), v.literal("abbezahlt"), v.literal("pausiert")),
   notes: v.optional(v.string()),
 });
@@ -32,7 +34,8 @@ function normalizeLoanFields(args: {
   monthlyPayment?: number;
   termMonths?: number;
 }) {
-  let { monthlyPayment, termMonths } = args;
+  let monthlyPayment = Number.isNaN(args.monthlyPayment ?? 0) ? undefined : args.monthlyPayment;
+  let termMonths = Number.isNaN(args.termMonths ?? 0) ? undefined : args.termMonths;
   if (monthlyPayment !== undefined && termMonths === undefined) {
     termMonths = computeTermFromPayment(args.principal, args.annualInterestRate, monthlyPayment);
   } else if (termMonths !== undefined && monthlyPayment === undefined) {
@@ -63,10 +66,13 @@ export const create = mutation({
     categoryId: v.optional(v.id("categories")),
     principal: v.number(),
     annualInterestRate: v.number(),
+    effectiveAnnualRate: v.optional(v.number()),
     monthlyPayment: v.optional(v.number()),
     termMonths: v.optional(v.number()),
     startDate: v.string(),
     currentBalance: v.optional(v.number()),
+    totalInterest: v.optional(v.number()),
+    totalAmount: v.optional(v.number()),
     status: v.union(v.literal("aktiv"), v.literal("abbezahlt"), v.literal("pausiert")),
     notes: v.optional(v.string()),
   },
@@ -82,10 +88,13 @@ export const create = mutation({
       categoryId: args.categoryId,
       principal: args.principal,
       annualInterestRate: args.annualInterestRate,
+      effectiveAnnualRate: args.effectiveAnnualRate,
       monthlyPayment: normalized.monthlyPayment,
       termMonths: normalized.termMonths,
       startDate: args.startDate,
       currentBalance: args.currentBalance,
+      totalInterest: args.totalInterest,
+      totalAmount: args.totalAmount,
       status: args.status,
       notes: args.notes,
     });
@@ -101,10 +110,13 @@ export const update = mutation({
     categoryId: v.optional(v.id("categories")),
     principal: v.optional(v.number()),
     annualInterestRate: v.optional(v.number()),
+    effectiveAnnualRate: v.optional(v.number()),
     monthlyPayment: v.optional(v.number()),
     termMonths: v.optional(v.number()),
     startDate: v.optional(v.string()),
     currentBalance: v.optional(v.number()),
+    totalInterest: v.optional(v.number()),
+    totalAmount: v.optional(v.number()),
     status: v.optional(
       v.union(v.literal("aktiv"), v.literal("abbezahlt"), v.literal("pausiert")),
     ),
@@ -133,8 +145,11 @@ export const update = mutation({
       "categoryId",
       "principal",
       "annualInterestRate",
+      "effectiveAnnualRate",
       "startDate",
       "currentBalance",
+      "totalInterest",
+      "totalAmount",
       "status",
       "notes",
     ] as const) {
@@ -163,7 +178,9 @@ export const computeSummary = query({
     currentBalance: v.number(),
     payoffDate: v.string(),
     totalInterest: v.number(),
+    totalAmount: v.number(),
     remainingMonths: v.number(),
+    paidMonths: v.number(),
   }),
   handler: async (ctx, args) => {
     const userId = await requireUserId(ctx);
@@ -175,10 +192,9 @@ export const computeSummary = query({
       startDate,
       monthlyPayment: loan.monthlyPayment,
       termMonths: loan.termMonths,
+      currentBalance: loan.currentBalance ?? undefined,
     });
-    const balance =
-      loan.currentBalance ??
-      currentBalanceFromSchedule(scheduleResult.schedule, startDate);
+    const balance = scheduleResult.currentBalance;
     const remainingMonths = scheduleResult.schedule.filter(
       (_, idx) =>
         idx >=
@@ -192,7 +208,9 @@ export const computeSummary = query({
       currentBalance: balance,
       payoffDate: scheduleResult.payoffDate.toISOString().slice(0, 10),
       totalInterest: scheduleResult.totalInterest,
+      totalAmount: scheduleResult.totalAmount,
       remainingMonths,
+      paidMonths: scheduleResult.paidMonths,
     };
   },
 });