MongoError: exception: After applying the update to the document {_id: ObjectId('55be3c8f79bae4f80c6b17f8') , ...}, the (immutable) field '_id' was found to have been altered to _id: ObjectId('55be5f15ae
5597401724aab3')

xmlFile.product_list.product.forEach(function (product) {

                    var productEntity = new Product({
                        product_id: parseInt(product.id),
                        parent_id: parseInt(product.parent_id),
                        sku: product.sku,
                        title: product.title,
                        pricenormal: parseFloat(product.price_normal),
                        pricewholesale: parseFloat(product.price_wholesale),
                        pricewholesalelarge: parseFloat(product.price_wholesale_large),
                        pricesale: parseFloat(product.price_sale),
                        weight: parseFloat(product.weight),
                        volume: parseFloat(product.volume),
                        unittype: product.unit_type,
                        status: product.status,
                        datecreating: product.datecreating,
                        sp: product.sp,
                        guaranteeext: product.guarantee_ext,
                        used: product.used,
                        statusfull: product.status_full,
                        description: null,
                        url: null
                    });
                    items.push(productEntity);

                });


items.forEach(function (product) { //update or create new products

                    Product.findOneAndUpdate({product_id: product.product_id}, product, {upsert: true}, function (err) {
                        if (err)  return updateDBCallback(err);
                        updateDBCallback(null, 'saved');

                    });

                });

//product._id = mongoose.Types.ObjectId(); //doesnt work
                //delete product._id // doesnt delete _id from product