How to prevent database getting previous old unupdated data - database

Question

I am trying to work with postgresQL with graphql in express server.
The problem I am facing is I want to update the database's value when user call the API in client. When it is called twice in short time, as the user's money and product's quantity is not yet updated, both function will get the some money, execute and the product's quantity and user's money will be negative which is not correct.
What is a simple implementation of the lock function so that that specific user's datarow and product's datarow cannot be access and modify until the previous process is done so that it is thread-safety?
Edit: I know I can added a lock in local cache, but I would like to ask what is the typical handle method? Such as I will just return the process is running when it is locked? Or I just create a loop to wait until first request is done? And if there is any good example of code that I can follow on.
#Mutation(() => Boolean)
async pay(#Arg('amount') amount: number) {
let userId = 1
let proeductId = 1
const user = await User.findOne(userId); // user = {userId: 1, money: 50}
const product = await Product.findOne(userId); // product= {userId:1, quantity: 1, price: 50}
await new Promise(resolve => setTimeout(resolve, 3000)) // some logic
if(user && product && user?.money >= product.price && product.quantity > 0 ){
await getConnection().getRepository(User).decrement({ userId }, 'money', 50);
await getConnection().getRepository(product).decrement({ userId }, 'quantity', 1);
}
}
}

Answer 1

Using .setLock("pessimistic_write") will solve the problem
const user= await getRepository(User)
.createQueryBuilder()
.useTransaction(true)
.setLock("pessimistic_write")
.where({id:userId})
.getOne();
const product = ...
if(user && product && user?.money >= product.price && product.quantity > 0 ){
await getRepository(User)
.createQueryBuilder()
.useTransaction(true)
.update(User)
.set({money: user.money- amount})
.where({id:userId})
.execute();
await getRepository(product)...
}