Node.js function that handles a wallet funding. Notice the atomic transaction

`

`async function fundWallet(userId: string, amount: Decimal, reference: string) {

return await prisma.$transaction(async (tx) => {

// 1. Check for duplicate

const existing = await tx.transaction.findUnique({ where: { reference } });

if (existing) return existing;

// 2. Lock wallet row (prevents race conditions)

const wallet = await tx.wallet.findUnique({

where: { userId },

});

if (!wallet || wallet.status !== 'ACTIVE') {

throw new Error('Wallet not available');

}

// 3. Create transaction record first

const newBalance = wallet.balance.plus(amount);

const transaction = await tx.transaction.create({

data: {

walletId: wallet.id,

type: 'CREDIT',

amount,

balanceAfter: newBalance,

reference,

status: 'COMPLETED',

description: 'Wallet funding via Paystack',

},

});

// 4. Update wallet balance

await tx.wallet.update({

where: { id: wallet.id },

data: { balance: newBalance },

});

return transaction;

});

}