[{$match: {
  status: 'completed'
}}, {$unwind: {
  path: '$products'
}}, {$group: {
  _id: {
    state: '$user.state',
    yearMonth: {
      $substr: [
        '$date_created',
        0,
        7
      ]
    },
    productcat: {
      $arrayElemAt: [
        '$products.category',
        1
      ]
    }
  },
  qtySold: {
    $sum: '$products.quantity'
  },
  totalRevenue: {
    $sum: '$products.total'
  },
  diffProducts: {
    $addToSet: '$products.name'
  }
}}, {$group: {
  _id: {
    state: '$_id.state',
    yearMonth: '$_id.yearMonth'
  },
  totalRevenue: {
    $sum: '$totalRevenue'
  },
  byCategory: {
    $push: {
      category: '$_id.productcat',
      qtySold: '$qtySold',
      revenue: '$totalRevenue',
      diffProducts: {
        $size: '$diffProducts'
      }
    }
  }
}}, {$project: {
  _id: 0,
  state: '$_id.state',
  yearMonth: '$_id.yearMonth',
  numCategories: {
    $size: '$byCategory'
  },
  salesByCategory: '$byCategory'
}}, {$sort: {
  state: 1,
  yearMonth: 1
}}]