Object id 数组匹配在 mongodb 聚合中查找 function

Question

I have an array of product ids and i want to use that array in lookup function to find the products, but i am getting an error in $in.

Cart Schema:

{
  "_id": "5ec7b6d484649a451f836102",
  "date": "2020-05-22T11:26:07.570Z",
  "checkout": false,
  "products": [
    {
      "_id": "5ec7b6d484649a451f836103",
      "product_id": "5ec7b1b184649a451f8360f8",
      "quantity": 1
    }
  ],
  "total_price": 150,
  "total_quantity": 1,
  "user_id": "5ec793d437b834429648657d",
  "price_with_discount": 148.5,
  "total_discount": 1.5,
  "__v": 0
}

Aggregation Function:

const checkouts = await Cart.aggregate([
            {
                $match: {
                    user_id, checkout: true
                }
            },
            {
                $group: {
                    _id: '$_id',
                    product_list: {
                        $first: '$products.product_id'
                    }
                }
            },
            {
                $lookup: {
                    from: 'products',
                    let: { list: "$product_list" },
                    pipeline: [
                        {
                            $match: {
                                _id: {
                                    $in: "$$list"
                                }
                            }
                        }
                    ],
                    as: 'cart_products',

                }
            }
        ]);

Getting the error that $in needs an array.

Answer 1

In order to use $$variable in a lookup, you have to wrap it in a $expr expression.

In the example you describe, $in is unnecessary, and you can use the simple form of lookup, which might be able to use an index:

 {$lookup: {
            from: 'products',
            localFields: "product_list",
            foreignField: "_id",        
            as: 'cart_products',
}}