Javascript 如何使用mongoose聚合根据字段获取两个匹配文档的总和

我有两个系列“员工”，“办公室”

我试图找出每个包含办公室代码的区域有多少员工。但同一地区可能有不止一个办公室

这就是我的办公室文件的样子

[
  {
    _id: "5b7d0f77e231b6b530b0ee5a",
    code: "OB123456",
    city: "Canmore"
    // some other fields            
  },
  {
    _id: "5b7d0f77e531b6b530b0ee5b",
    code: "OB858758",
    city: "Vancouver"
  },
  {
    _id: "5b7d0f77e531b6b530b0ee5d",
    code: "EE858758",
    city: "Vancouver"
  },
]

[
  {
    _id: "5b7d0f77e531b6b530b0edda",
    name: 'Charlie',
    office: {
      code: 'OB123456'
      // some other fields
    }
  },
  {
    _id: "5b7d0f73e531b6b530b0ee5b",
    name: 'Bill',
    office: {
      code: 'EE858758'
    }
  },
  {
    _id: "5b7d0f77e531b6b530b0ee5n",
    name: 'Echo',
    office: {
      code: 'OB123456'
    }
  },
];

这就是我的员工文档的外观

[
  {
    _id: "5b7d0f77e231b6b530b0ee5a",
    code: "OB123456",
    city: "Canmore"
    // some other fields            
  },
  {
    _id: "5b7d0f77e531b6b530b0ee5b",
    code: "OB858758",
    city: "Vancouver"
  },
  {
    _id: "5b7d0f77e531b6b530b0ee5d",
    code: "EE858758",
    city: "Vancouver"
  },
]

[
  {
    _id: "5b7d0f77e531b6b530b0edda",
    name: 'Charlie',
    office: {
      code: 'OB123456'
      // some other fields
    }
  },
  {
    _id: "5b7d0f73e531b6b530b0ee5b",
    name: 'Bill',
    office: {
      code: 'EE858758'
    }
  },
  {
    _id: "5b7d0f77e531b6b530b0ee5n",
    name: 'Echo',
    office: {
      code: 'OB123456'
    }
  },
];

我正在调查猫鼬的总数，并且只做了尝试

await Employee.aggregate([
      {
        $lookup: {
          from: 'offices',
          localField: 'office.code',
          foreignField: 'code',
          as: 'officeCode'
        },
        $group: {
          _id: 'officeCode.city',
          count: { $sum: 1 }
        }
      }
    ]);

这肯定不起作用，我试着阅读了一些聚合文档，但没有想出一个好主意如何完成这项工作

提前感谢您的任何建议或建议

我正在寻找的样本输出

{
    "Vancouver": 1,
    "Canmore": 2
}

---

您必须从办公室开始，而不是从员工开始，这样您就可以为每个地区（城市）创建代码列表，然后查找以与您的员工进行映射

db.office.aggregate([
  {
    $group: {
      _id: "$city",
      codes: {
        $addToSet: "$code"
      }
    }
  },
  {
    $lookup: {
      from: "employee",
      localField: "codes",
      foreignField: "office.code",
      as: "employees"
    },
    
  },
  {
    $group: {
      _id: null,
      data: {
        $push: {
          k: "$_id",
          v: {
            $size: "$employees"
          }
        }
      }
    }
  },
  {
    $replaceRoot: {
      newRoot: {
        "$arrayToObject": "$data"
      }
    }
  }
])

最后两个阶段仅用于按照预期输出中的描述格式化结果