如何在 MongoDB Aggregate 中查找内部查找？答案

【问题标题】：How to lookup inside lookup in MongoDB Aggregate?如何在 MongoDB Aggregate 中查找内部查找？
【发布时间】：2021-02-24 12:25:51
【问题描述】：

我有一个简单的 3 个集合。下面是他们的伪代码。我想获得所有货物，对于每批货物，我想获得该货物的所有出价，并且在每次出价中，我都需要 userDetails 对象。

User: {
  name: string,
}

Shipment: {
  from: string,
  to: string
}

Bid: {
  amount: number,
  shipmentId: Ref_to_Shipment
  userId: Ref_to_User
}

这是我尝试过的：

const shipments = await ShipmentModel.aggregate([
            {
                $lookup: {
                    from: "bids",
                    localField: "_id",
                    foreignField: "shipmentId",
                    as: "bids"
                }
            },
            {
                $lookup: {
                    from: "users",
                    localField: "bids.userId",
                    foreignField: "_id",
                    as: "bids.user"
                }
            }
        ])

我得到了以下结果：

[
    {
        "_id": "5fad2fc04458ac156531d1b1",
        "from": "Belgrade",
        "to": "London",
        "__v": 0,
        "bids": {
            "user": [
                {
                    "_id": "5fad2cdb4d19c80d1b6abcb7",
                    "name": "Amel",
                    "email": "Muminovic",
                    "password": "d2d2d2",
                    "__v": 0
                }
            ]
        }
    }
]

我正在尝试获取所有货件及其出价和出价内的用户。 数据应如下所示：

[
    {
        "_id": "5fad2fc04458ac156531d1b1",
        "from": "Belgrade",
        "to": "London",
        "__v": 0,
        "bids": [
            {
                "_id": "5fad341887c2ae1feff73402",
                "amount": 400,
                "userId": "5fad2cdb4d19c80d1b6abcb7",
                "shipmentId": "5fad2fc04458ac156531d1b1",
                "user": {
                    "name": "Amel",
                }
                "__v": 0
            }
        ]
    }
]

【问题讨论】：

标签： node.js database mongodb mongoose

【解决方案1】：

试试下面的代码：

const shipments = await ShipmentModel.aggregate([
            {
                $lookup: {
                    from: "bids",
                    localField: "_id",
                    foreignField: "shipmentId",
                    as: "bids"
                }
            },
            {
              $unwind: {
                path: "$bids",
                preserveNullAndEmptyArrays: true
              }
            },
            {
                $lookup: {
                    from: "users",
                    localField: "bids.userId",
                    foreignField: "_id",
                    as: "bids.user"
                }
            }
        ])

如果你想防止 null 和空数组然后设置

preserveNullAndEmptyArrays: false

【讨论】：

这样我就可以为每个未平仓出价获得发货。我可以在一个 Shipment 中有 10 个投标，所以我将得到 10 个以投标作为对象而不是数组的货件？

【解决方案2】：

尝试this 查询并检查是否有效并且行为是否符合您的预期：

db.Shipment.aggregate([
  {
    $lookup: {
      from: "Bid",
      localField: "id",
      foreignField: "shipmentId",
      as: "bids"
    }
  },
  {
    $lookup: {
      from: "user",
      localField: "id",
      foreignField: "id",
      as: "newBids"
    }
  },
  {
    $project: {
      "newBids.id": 0,
      "newBids._id": 0,
      
    }
  },
  {
    $match: {
      "bids.userId": 1
    }
  },
  {
    $addFields: {
      "newBids": {
        "$arrayElemAt": [
          "$newBids",
          0
        ]
      }
    }
  },
  {
    $set: {
      "bids.user": "$newBids"
    }
  },
  {
    $project: {
      "newBids": 0
    }
  }
])

此查询执行您的双重$lookup，然后执行$project 以删除您不需要的字段，并查找userId 以添加字段user。由于$lookup 生成一个数组，所以必须使用arrayElemAt 来获取第一个位置。

然后将$set这个值生成到对象中作为bids.user并删除旧值。

请注意，我使用了一个新字段 id 而不是 _id 来更轻松地读取数据。

【讨论】：