feat(transfer): implement transfer functionality with user selection and balance validation

src/app/api/transfer/route.ts

@@ -0,0 +1,75 @@
+import { NextResponse } from "next/server";
+import { auth } from "~/server/auth";
+import { db } from "~/server/db";
+
+export async function POST(request: Request) {
+  const session = await auth();
+
+  if (!session?.user?.id) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  const { toUserId, amount } = (await request.json()) as {
+    toUserId: string;
+    amount: number;
+  };
+
+  if (!toUserId || typeof toUserId !== "string") {
+    return NextResponse.json({ error: "Invalid recipient" }, { status: 400 });
+  }
+
+  if (typeof amount !== "number" || isNaN(amount) || amount <= 0) {
+    return NextResponse.json(
+      { error: "Amount must be a positive number" },
+      { status: 400 },
+    );
+  }
+
+  const fromUserId = session.user.id;
+
+  if (fromUserId === toUserId) {
+    return NextResponse.json(
+      { error: "Cannot transfer to yourself" },
+      { status: 400 },
+    );
+  }
+
+  const [sender, recipient] = await Promise.all([
+    db.user.findUnique({
+      where: { id: fromUserId },
+      select: { id: true, balance: true },
+    }),
+    db.user.findUnique({
+      where: { id: toUserId },
+      select: { id: true, name: true },
+    }),
+  ]);
+
+  if (!sender) {
+    return NextResponse.json({ error: "Sender not found" }, { status: 404 });
+  }
+
+  if (!recipient) {
+    return NextResponse.json({ error: "Recipient not found" }, { status: 404 });
+  }
+
+  if (sender.balance < amount) {
+    return NextResponse.json(
+      { error: "Insufficient balance" },
+      { status: 400 },
+    );
+  }
+
+  await db.$transaction([
+    db.user.update({
+      where: { id: fromUserId },
+      data: { balance: { decrement: amount } },
+    }),
+    db.user.update({
+      where: { id: toUserId },
+      data: { balance: { increment: amount } },
+    }),
+  ]);
+
+  return NextResponse.json({ success: true, amount, to: recipient.name });
+}

src/app/api/users/route.ts

@@ -0,0 +1,19 @@
+import { NextResponse } from "next/server";
+import { auth } from "~/server/auth";
+import { db } from "~/server/db";
+
+export async function GET() {
+  const session = await auth();
+
+  if (!session?.user?.id) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  const users = await db.user.findMany({
+    where: { id: { not: session.user.id } },
+    select: { id: true, name: true },
+    orderBy: { name: "asc" },
+  });
+
+  return NextResponse.json(users);
+}