在MongoDB中使用嵌套分组进行堆叠

{
  $group: {
    _id: { <expression> },
    <field1>: { <accumulator1> : <expression1> },
    ...
  }
}

db.sales.aggregate([
  {
    $group: {
      _id: {
        region: "$region",
        year: { $year: "$date" },
        month: { $month: "$date" }
      },
      totalSales: { $sum: "$amount" }
    }
  },
  {
    $group: {
      _id: "$_id.region",
      monthlySales: {
        $push: {
          year: "$_id.year",
          month: "$_id.month",
          sales: "$totalSales"
        }
      }
    }
  }
])