getTotalAmount: (userId,products) => {    
    return new Promise(async(res,rej) => {
        let total = await db.get().collection(collections.CART_COLLECTION).aggregate([
            {
                $match:{user: ObjectId(userId)}
            },
            {
                $unwind:'$products'
            },
            {
                $project:{
                    item: '$products.item',
                    quantity: '$products.quantity'
                }
            },
            {
                $lookup:{
                    from: collections.PRODUCT_COLLECTION,
                    localField: 'item',
                    foreignField: '_id',
                    as: 'product'
                }
            },
            {
                $project:{
                    item:1,quantity:1,product:{$arrayElemAt:["$product",0]}
                }
            },
            {
                $group: { 
                    _id: null,
                    total: {
                        $sum: {
                            $cond: {
                                if: {$ifNull: [ "$product.OfferPrice", false ]},
                                then: {$multiply: [{ $toDecimal: '$quantity' }, {$convert: {input: {$ifNull: ["$product.OfferPrice", 0]}, to: "double"}}]},
                                else: {$multiply: [{ $toDecimal: '$quantity' }, {$convert: {input: { $ifNull: ["$product.Price", 0] }, to: "double"}}]}
                            }
                        }
                    }
                }
            }
        ]).toArray()
        // console.log(total)
            res(total[0].total) 
    })

回调（新错误_1.MongoServerError（文档））;^
服务器错误：汇总期间PlanExecutor错误：：原因：无法解析$convert中的数字""，没有onError值：空字符串