Next.js プロジェクトを大規模に構造化する：痛い目に遭って学んだこと

// features/invoicing/hooks/use-invoice-actions.ts
import { useCallback } from "react";
import { Invoice } from "../types";
import { formatInvoiceNumber } from "../utils/format";
import { invoiceApi } from "../api/client";
 
export function useInvoiceActions(invoice: Invoice) {
  const markAsPaid = useCallback(async () => {
    await invoiceApi.updateStatus(invoice.id, "paid");
  }, [invoice.id]);
 
  const duplicate = useCallback(async () => {
    const newNumber = formatInvoiceNumber(Date.now());
    await invoiceApi.create({ ...invoice, number: newNumber, status: "draft" });
  }, [invoice]);
 
  return { markAsPaid, duplicate };
}

// app/(dashboard)/layout.tsx
import { redirect } from "next/navigation";
import { getSession } from "@/shared/lib/auth";
import { Sidebar } from "@/shared/components/layout/sidebar";
 
export default async function DashboardLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  const session = await getSession();
 
  if (!session) {
    redirect("/login");
  }
 
  return (
    <div className="flex min-h-screen">
      <Sidebar user={session.user} />
      <main className="flex-1 p-6">{children}</main>
    </div>
  );
}

// app/(public)/layout.tsx
import { Header } from "@/shared/components/layout/header";
import { Footer } from "@/shared/components/layout/footer";
 
export default function PublicLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  return (
    <>
      <Header />
      <main>{children}</main>
      <Footer />
    </>
  );
}

// shared/lib/db.ts — これは shared に属する
import { drizzle } from "drizzle-orm/neon-http";
import { neon } from "@neondatabase/serverless";
 
const sql = neon(process.env.DATABASE_URL!);
export const db = drizzle(sql);
 
// features/invoicing/utils/format.ts — これは shared に属さない
export function formatInvoiceNumber(timestamp: number): string {
  const date = new Date(timestamp);
  const year = date.getFullYear();
  const seq = Math.floor(Math.random() * 9000) + 1000;
  return `INV-${year}-${seq}`;
}

// これは合理的に見えるが間違いだ
"use client";
 
import { InvoiceTable } from "@/features/invoicing/components/invoice-table";
import { InvoiceFilters } from "@/features/invoicing/components/invoice-filters";
import { useInvoiceActions } from "@/features/invoicing/hooks/use-invoice-actions";
 
export default function InvoicesPage() {
  const { data } = useInvoiceActions();
  return (
    <div>
      <InvoiceFilters />
      <InvoiceTable invoices={data} />
    </div>
  );
}

// app/(dashboard)/invoices/page.tsx — サーバーコンポーネント（ディレクティブなし）
import { db } from "@/shared/lib/db";
import { InvoiceTable } from "@/features/invoicing/components/invoice-table";
import { InvoiceFilters } from "@/features/invoicing/components/invoice-filters";
 
export default async function InvoicesPage() {
  const invoices = await db.query.invoices.findMany({
    orderBy: (inv, { desc }) => [desc(inv.createdAt)],
  });
 
  return (
    <div>
      <InvoiceFilters />          {/* クライアントコンポーネント — インタラクティブなフィルターを持つ */}
      <InvoiceTable data={invoices} /> {/* サーバーコンポーネント — 行をレンダリングするだけ */}
    </div>
  );
}

// features/invoicing/components/invoice-filters.tsx
"use client";
 
import { useRouter, useSearchParams } from "next/navigation";
 
export function InvoiceFilters() {
  const router = useRouter();
  const params = useSearchParams();
 
  const setStatus = (status: string) => {
    const next = new URLSearchParams(params.toString());
    next.set("status", status);
    router.push(`?${next.toString()}`);
  };
 
  return (
    <div className="flex gap-2">
      {["all", "draft", "sent", "paid"].map((s) => (
        <button
          key={s}
          onClick={() => setStatus(s)}
          className={params.get("status") === s ? "font-bold" : ""}
        >
          {s}
        </button>
      ))}
    </div>
  );
}

"use client";
 
export function InteractiveWrapper({ children }: { children: React.ReactNode }) {
  const [isOpen, setIsOpen] = useState(false);
  return (
    <div>
      <button onClick={() => setIsOpen(!isOpen)}>Toggle</button>
      {isOpen && children}  {/* children はサーバーコンポーネントにできる */}
    </div>
  );
}

// features/invoicing/components/index.ts
export { InvoiceTable } from "./invoice-table";
export { InvoiceRow } from "./invoice-row";
export { InvoiceForm } from "./invoice-form";
export { InvoicePdf } from "./invoice-pdf";

import { InvoiceTable } from "@/features/invoicing/components";

// こうする
import { InvoiceTable } from "@/features/invoicing/components/invoice-table";
 
// こうしない
import { InvoiceTable } from "@/features/invoicing/components";

// app/(dashboard)/invoices/[id]/page.tsx
import { notFound } from "next/navigation";
import { db } from "@/shared/lib/db";
import { InvoiceDetail } from "@/features/invoicing/components/invoice-detail";
 
interface Props {
  params: Promise<{ id: string }>;
}
 
export default async function InvoicePage({ params }: Props) {
  const { id } = await params;
  const invoice = await db.query.invoices.findFirst({
    where: (inv, { eq }) => eq(inv.id, id),
  });
 
  if (!invoice) notFound();
 
  return <InvoiceDetail invoice={invoice} />;
}