Grouping by date

Reporting questions are almost always date questions - per day, per month, per year. There are two ways to group by date, and the choice between them is really a choice about what you want the output label to look like.

You will almost certainly encounter:

Group sales by month.

Suppose:

{
  createdAt: ISODate("2026-01-15"),
  amount: 500
}

You can group by year and month:

{
  $group: {
    _id: {
      year: {
        $year: "$createdAt"
      },

      month: {
        $month: "$createdAt"
      }
    },

    revenue: {
      $sum: "$amount"
    }
  }
}

Result:

{
  _id: {
    year: 2026,
    month: 1
  },

  revenue: 5000
}

$dateToString

Often cleaner for output:

{
  $group: {
    _id: {
      $dateToString: {
        format: "%Y-%m",
        date: "$createdAt"
      }
    },

    revenue: {
      $sum: "$amount"
    }
  }
}

Result:

{
  _id: "2026-01",
  revenue: 5000
}

Then:

{
  $sort: {
    _id: 1
  }
}

Try it

db.orders.aggregate([
  {
    $group: {
      _id: {
        year: { $year: "$createdAt" },
        month: { $month: "$createdAt" }
      },
      orders: { $sum: 1 }
    }
  },
  { $sort: { "_id.year": 1, "_id.month": 1 } }
])

The label is an object, so sorting means reaching through _id. Try $dateToString instead and the label becomes "2025-11" - one string, sortable on its own.