Request Handling at backend (Django)

I have two api endpoints
1- stockpurchases/ || stockpurchases/{id}/
2- stockpurchases/{id}/stocks/ || stockpurchases/{id}/stocks/{id}/

So if some one want to purchase he should create a stockpurchase first and than add a stock to it and when someone POST stock I plus quantity and amount of stockpurchase.

The problem is if some one call POST request consecutively for a loop the amount operation will goes incorrect.

I want handle multiple request at the backend. Supose if the server gets consecutive request to perform operation how can i make others request wait till completion of first procedsed. After that second request is processed and so on.

StockPurchase Model

class StockPurchase(models.Model):
    qty = models.IntegerField(default=0)
    amount = models.IntegerField(default=0)

Stock Model

class Stock(models.Model):
    stockpurchase = models.ForeignKey("StockPurchase", on_delete=models.CASCADE, related_name='stocks')
    product = models.ForeignKey("Product", on_delete=models.CASCADE, related_name='stocks')
    project = models.ForeignKey("Project", on_delete=models.CASCADE, related_name='stocks')

    rate = models.IntegerField()

     def save(self, *args, **kwargs):
        if not
            self.stockpurchase.amount += self.rate
            self.stockpurchase.qty += 1

So if I Call like this with await no issue

async function create() {
  for (let i = 0; i < 10; i++) {
    let response = await fetch(
        method: "POST",
        body: bodyContent,
        headers: headersList,

but if call it without await async function the amount of stockpurchase will be wrong like this

for (let i = 0; i < 10; i++) {
  let response = fetch("http://localhost:8000/api/stockpurchases/2/stocks/", {
    method: "POST",
    body: bodyContent,
    headers: headersList,

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