How to pass element index in array to mongoDB query?

Question

Building cart on website and when product is added i want to first check if it is already in cart, if yes increment quantity by 1, if not add it. Cart is an array of objects and i want to pass index of object that contains added product to increment function but can't figure out how to do so.

async function add(product, userId) {
      const user = await User.findById(userId);
      const product = isProductInCart(product, user.cart); // returns true and index in cart if found
      if (product.found === true) {
        await User.findOneAndUpdate(
          { _id: userId },
          { $inc: { cart[product.index].quantity : 1 }} // not working
        );
      } else {
        await User.findOneAndUpdate({ _id: userId }, { $push: { cart: product } });
      }
    }

function isProductInCart(product, cart) {
  let productFound = { found: false, index: -1 };
  for (let i = 0; i < cart.length; i++)
    if (cart[i].name === product.name) {
      productFound.found = true;
      productFound.index = i;
      break;
    }
  return productFound;
}

Can you show the isProductInCart function?

mickl
– mickl

2020-07-16 13:34:41 +00:00
Commented Jul 16, 2020 at 13:34 — mickl
– mickl, Commented Jul 16, 2020 at 13:34

mickl · Accepted Answer · 2020-07-16 13:53:09Z

1

It looks like your code can be simplified if you consider using the $ positional operator:

let userWithCart = User.findOneAndUpdate(
    { _id: user, 'cart.name': product.name },
    { $inc: { 'cart.$.quantity' : 1 }}
)

if(!userWithCart ){
    await User.findOneAndUpdate({ _id: userId }, { $push: { cart: product } });
}

First findOneAndUpdate will return no value when there's no corresponding cart.name (and you need to $push it). Otherwise MongoDB will automatically match the cart you want to update based on cart.name condition and increment the quantity subfield.

EDIT: If you still need to proceed the way you've started you just need to evaluate the path in JavaScript:

{ $inc: { [`cart.${product.index}.quantity`] : 1 }}

edited Jul 16, 2020 at 13:53

answered Jul 16, 2020 at 13:44

mickl

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