First version

src/lib/api.ts

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+import { getPrisma } from '@/lib/prisma'
+import { ExpenseFormValues, GroupFormValues } from '@/lib/schemas'
+import { Expense } from '@prisma/client'
+import { v4 as uuidv4 } from 'uuid'
+
+export async function createGroup(groupFormValues: GroupFormValues) {
+  return getPrisma().group.create({
+    data: {
+      id: uuidv4(),
+      name: groupFormValues.name,
+      participants: {
+        createMany: {
+          data: groupFormValues.participants.map(({ name }) => ({
+            id: uuidv4(),
+            name,
+          })),
+        },
+      },
+    },
+    include: { participants: true },
+  })
+}
+
+export async function createExpense(
+  expenseFormValues: ExpenseFormValues,
+  groupId: string,
+): Promise<Expense> {
+  const group = await getGroup(groupId)
+  if (!group) throw new Error(`Invalid group ID: ${groupId}`)
+
+  for (const participant of [
+    expenseFormValues.paidBy,
+    ...expenseFormValues.paidFor,
+  ]) {
+    if (!group.participants.some((p) => p.id === participant))
+      throw new Error(`Invalid participant ID: ${participant}`)
+  }
+
+  return getPrisma().expense.create({
+    data: {
+      id: uuidv4(),
+      groupId,
+      amount: expenseFormValues.amount,
+      title: expenseFormValues.title,
+      paidById: expenseFormValues.paidBy,
+      paidFor: {
+        createMany: {
+          data: expenseFormValues.paidFor.map((paidFor) => ({
+            participantId: paidFor,
+          })),
+        },
+      },
+    },
+  })
+}
+
+export async function updateExpense(
+  groupId: string,
+  expenseId: string,
+  expenseFormValues: ExpenseFormValues,
+) {
+  const group = await getGroup(groupId)
+  if (!group) throw new Error(`Invalid group ID: ${groupId}`)
+
+  const existingExpense = await getExpense(groupId, expenseId)
+  if (!existingExpense) throw new Error(`Invalid expense ID: ${expenseId}`)
+
+  for (const participant of [
+    expenseFormValues.paidBy,
+    ...expenseFormValues.paidFor,
+  ]) {
+    if (!group.participants.some((p) => p.id === participant))
+      throw new Error(`Invalid participant ID: ${participant}`)
+  }
+
+  return getPrisma().expense.update({
+    where: { id: expenseId },
+    data: {
+      amount: expenseFormValues.amount,
+      title: expenseFormValues.title,
+      paidById: expenseFormValues.paidBy,
+      paidFor: {
+        connectOrCreate: expenseFormValues.paidFor.map((paidFor) => ({
+          where: {
+            expenseId_participantId: { expenseId, participantId: paidFor },
+          },
+          create: { participantId: paidFor },
+        })),
+        deleteMany: existingExpense.paidFor.filter(
+          (paidFor) =>
+            !expenseFormValues.paidFor.some(
+              (pf) => pf === paidFor.participantId,
+            ),
+        ),
+      },
+    },
+  })
+}
+
+export async function updateGroup(
+  groupId: string,
+  groupFormValues: GroupFormValues,
+) {
+  const existingGroup = await getGroup(groupId)
+  if (!existingGroup) throw new Error('Invalid group ID')
+
+  return getPrisma().group.update({
+    where: { id: groupId },
+    data: {
+      name: groupFormValues.name,
+      participants: {
+        deleteMany: existingGroup.participants.filter(
+          (p) => !groupFormValues.participants.some((p2) => p2.id === p.id),
+        ),
+        updateMany: groupFormValues.participants
+          .filter((participant) => participant.id !== undefined)
+          .map((participant) => ({
+            where: { id: participant.id },
+            data: {
+              name: participant.name,
+            },
+          })),
+        createMany: {
+          data: groupFormValues.participants
+            .filter((participant) => participant.id === undefined)
+            .map((participant) => ({
+              id: uuidv4(),
+              name: participant.name,
+            })),
+        },
+      },
+    },
+  })
+}
+
+export async function getGroup(groupId: string) {
+  return getPrisma().group.findUnique({
+    where: { id: groupId },
+    include: { participants: true },
+  })
+}
+
+export async function getGroupExpenses(groupId: string) {
+  return getPrisma().expense.findMany({
+    where: { groupId },
+    include: { paidFor: { include: { participant: true } }, paidBy: true },
+  })
+}
+
+export async function getExpense(groupId: string, expenseId: string) {
+  return getPrisma().expense.findUnique({
+    where: { id: expenseId },
+    include: { paidBy: true, paidFor: true },
+  })
+}

src/lib/prisma.ts

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+import { PrismaClient } from '@prisma/client'
+
+let prisma: PrismaClient
+
+export function getPrisma() {
+  if (!prisma) {
+    if (process.env.NODE_ENV === 'production') {
+      prisma = new PrismaClient()
+    } else {
+      if (!(global as any).prisma) {
+        ;(global as any).prisma = new PrismaClient()
+      }
+      prisma = (global as any).prisma
+    }
+  }
+  return prisma
+}

src/lib/schemas.ts

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+import * as z from 'zod'
+
+export const groupFormSchema = z
+  .object({
+    name: z
+      .string()
+      .min(2, 'Enter at least two characters.')
+      .max(50, 'Enter at most 50 characters.'),
+    participants: z
+      .array(
+        z.object({
+          id: z.string().optional(),
+          name: z
+            .string()
+            .min(2, 'Enter at least two characters.')
+            .max(50, 'Enter at most 50 characters.'),
+        }),
+      )
+      .min(1),
+  })
+  .superRefine(({ participants }, ctx) => {
+    participants.forEach((participant, i) => {
+      participants.slice(0, i).forEach((otherParticipant) => {
+        if (otherParticipant.name === participant.name) {
+          ctx.addIssue({
+            code: 'custom',
+            message: 'Another participant already has this name.',
+            path: ['participants', i, 'name'],
+          })
+        }
+      })
+    })
+  })
+
+export type GroupFormValues = z.infer<typeof groupFormSchema>
+
+export const expenseFormSchema = z.object({
+  title: z
+    .string({ required_error: 'Please enter a title.' })
+    .min(2, 'Enter at least two characters.'),
+  amount: z.coerce
+    .number({ required_error: 'You must enter an amount.' })
+    .min(0.01, 'The amount must be higher than 0.01.'),
+  paidBy: z.string({ required_error: 'You must select a participant.' }),
+  paidFor: z
+    .array(z.string())
+    .min(1, 'The expense must be paid for at least 1 participant.'),
+})
+
+export type ExpenseFormValues = z.infer<typeof expenseFormSchema>

src/lib/utils.ts

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+import { type ClassValue, clsx } from "clsx"
+import { twMerge } from "tailwind-merge"
+ 
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs))
+}