I am working on aggregation to perform some analytics on an Orders object, I want to get the total price from every order from the previous month, and the total from this month, I tried passing in the $match parameter, with the appropriate time conditionals but it's not aggregating any of the data. even though if I perform a countDocuments query with the same time conditionals I get a number of documents returned.

const totalThis = await Order.aggregate([
  {
    $match: {
      // should cut the amount of orders off at the beginning of the current month
      createdAt: { $gte: thisMonth.getMonth()   1 },
    },
  },
  {
    $group: {
      // using $group, _id has to be there or it doesnt
      // return anything, though we dont need an _id
      _id: null,
      total: {
        $sum: "$totalPrice",
      },
    },
  },
]);
res.status(200).json({
  lastMonth: lastMonthSales,
  thisMonth: thisMonthSales,
  totalSalesLast: totalLast,
  totalSalesThis: totalThis,
});

In my Postman request, lastMonth returns 0, so it would make sense that the array returned to totalSalesLast would be empty, however thisMonth returns 5 documents, so it should work. Obviously, if I remove the $match conditional it will aggregate all the documents, but that's because it's doing ALL the documents and not actually filtering.

CodePudding user response：

To query the document with createdAt in specific month, your $match stage should be as below:

1. $month - Take the month value from createdAt.
2. $eq - Compare both month from (1) and input month are same.

{
  $match: {
    $expr: {
      $eq: [
        {
          "$month": "$createdAt"
        },
        thisMonth.getMonth()   1
      ]
    }
  }
}

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