Use decimal.js to validate uneven amounts

* Update expense-form.tsx to handle shares as strings

Proposing fix to #424

The issue is in the data flow between the form and the schema transform function:

When editing existing expenses: Form loads shares by dividing database values by 100 (e.g., 200 / 100 = 2), but loads them as numbers
When users change values: Input fields return strings via enforceCurrencyPattern
Schema transform: Only multiplies by 100 for string values, not number values
Result: Modified shares (strings) get multiplied by 100, unmodified shares (numbers) stay as-is

Proposed fix: handle all shares consistently as strings throughout the form

* Add type assertions to fix TypeScript errors in expense form

Fix formatting.

---------

Co-authored-by: yllar <yllar.pajus@gmail.com>

prisma/migrations/20251103161018_add_ondelete_cascade_to_activity_groupid/migration.sql

@@ -0,0 +1,5 @@
+-- DropForeignKey
+ALTER TABLE "Activity" DROP CONSTRAINT "Activity_groupId_fkey";
+
+-- AddForeignKey
+ALTER TABLE "Activity" ADD CONSTRAINT "Activity_groupId_fkey" FOREIGN KEY ("groupId") REFERENCES "Group"("id") ON DELETE CASCADE ON UPDATE CASCADE;

prisma/schema.prisma

@@ -116,7 +116,7 @@ model ExpensePaidFor {
 
 model Activity {
   id            String       @id
-  group         Group        @relation(fields: [groupId], references: [id])
+  group         Group        @relation(fields: [groupId], references: [id], onDelete: Cascade)
   groupId       String
   time          DateTime     @default(now())
   activityType  ActivityType

src/app/groups/[groupId]/expenses/expense-form.tsx

@@ -81,7 +81,7 @@ const getDefaultSplittingOptions = (
     splitMode: 'EVENLY' as const,
     paidFor: group.participants.map(({ id }) => ({
       participant: id,
-      shares: 1,
+      shares: '1' as any, // Use string to ensure consistent schema handling
     })),
   }
 
@@ -113,7 +113,7 @@ const getDefaultSplittingOptions = (
     splitMode: parsedDefaultSplitMode.splitMode,
     paidFor: parsedDefaultSplitMode.paidFor.map((paidFor) => ({
       participant: paidFor.participant,
-      shares: paidFor.shares / 100,
+      shares: (paidFor.shares / 100).toString() as any, // Convert to string for consistent schema handling
     })),
   }
 }
@@ -197,10 +197,9 @@ export function ExpenseForm({
           paidBy: expense.paidById,
           paidFor: expense.paidFor.map(({ participantId, shares }) => ({
             participant: participantId,
-            shares:
-              expense.splitMode === 'BY_AMOUNT'
-                ? amountAsDecimal(shares, groupCurrency)
-                : shares / 100,
+            shares: (expense.splitMode === 'BY_AMOUNT'
+              ? amountAsDecimal(shares, groupCurrency)
+              : (shares / 100).toString()) as any, // Convert to string to ensure consistent handling
           })),
           splitMode: expense.splitMode,
           saveDefaultSplittingOptions: false,
@@ -226,7 +225,7 @@ export function ExpenseForm({
             searchParams.get('to')
               ? {
                   participant: searchParams.get('to')!,
-                  shares: 1,
+                  shares: '1' as any, // String for consistent form handling
                 }
               : undefined,
           ],
@@ -359,9 +358,9 @@ export function ExpenseForm({
           if (!editedParticipants.includes(participant.participant)) {
             return {
               ...participant,
-              shares: Number(
-                amountPerRemaining.toFixed(groupCurrency.decimal_digits),
-              ),
+              shares: amountPerRemaining.toFixed(
+                groupCurrency.decimal_digits,
+              ) as any, // Keep as string for consistent schema handling
             }
           }
           return participant
@@ -825,11 +824,11 @@ export function ExpenseForm({
                     ? []
                     : group.participants.map((p) => ({
                         participant: p.id,
-                        shares:
-                          paidFor.find((pfor) => pfor.participant === p.id)
-                            ?.shares ?? 1,
+                        shares: (paidFor.find(
+                          (pfor) => pfor.participant === p.id,
+                        )?.shares ?? '1') as any, // Use string to ensure consistent schema handling
                       }))
-                  form.setValue('paidFor', newPaidFor, {
+                  form.setValue('paidFor', newPaidFor as any, {
                     shouldDirty: true,
                     shouldTouch: true,
                     shouldValidate: true,
@@ -886,9 +885,9 @@ export function ExpenseForm({
                                             ...field.value,
                                             {
                                               participant: id,
-                                              shares: 1,
+                                              shares: '1', // Use string to ensure consistent schema handling
                                             },
-                                          ],
+                                          ] as any,
                                           options,
                                         )
                                       : form.setValue(

src/lib/schemas.ts

@@ -1,4 +1,6 @@
 import { RecurrenceRule, SplitMode } from '@prisma/client'
+import Decimal from 'decimal.js'
+
 import * as z from 'zod'
 
 export const groupFormSchema = z
@@ -148,14 +150,14 @@ export const expenseFormSchema = z
         break // noop
       case 'BY_AMOUNT': {
         const sum = expense.paidFor.reduce(
-          (sum, { shares }) => sum + Number(shares),
-          0,
+          (sum, { shares }) => new Decimal(shares).add(sum),
+          new Decimal(0),
         )
-        if (sum !== expense.amount) {
-          const detail =
-            sum < expense.amount
-              ? `${((expense.amount - sum) / 100).toFixed(2)} missing`
-              : `${((sum - expense.amount) / 100).toFixed(2)} surplus`
+        if (!sum.equals(new Decimal(expense.amount))) {
+          // const detail =
+          //   sum < expense.amount
+          //     ? `${((expense.amount - sum) / 100).toFixed(2)} missing`
+          //     : `${((sum - expense.amount) / 100).toFixed(2)} surplus`
           ctx.addIssue({
             code: z.ZodIssueCode.custom,
             message: 'amountSum',