如何使用聚合管道提取MongoDB中所有子文档的特定字段？

Question

我有一个 Mongo 集合

customer

，其结构如下。

现在我正在尝试将

tier

的所有子文档的

tier_and_details

字段的值提取到一个数组中。

{
  "_id": ObjectId("5ca4bbcea2dd94ee58162a69"),
  "username": "valenciajennifer",
  "name": "Lindsay Cowan",
  "address": "Unit 1047 Box 4089\nDPO AA 57348",
  "birthdate": ISODate("1994-02-19T23:46:27Z"),
  "email": "[email protected]",
  "accounts": [
    116508
  ],
  "tier_and_details": {
    "c06d340a4bad42c59e3b6665571d2907": {
      "tier": "Platinum",
      "benefits": [
        "dedicated account representative"
      ],
      "active": true,
      "id": "c06d340a4bad42c59e3b6665571d2907"
    },
    "5d6a79083c26402bbef823a55d2f4208": {
      "tier": "Bronze",
      "benefits": [
        "car rental insurance",
        "concierge services"
      ],
      "active": true,
      "id": "5d6a79083c26402bbef823a55d2f4208"
    },
    "b754ec2d455143bcb0f0d7bd46de6e06": {
      "tier": "Gold",
      "benefits": [
        "airline lounge access"
      ],
      "active": true,
      "id": "b754ec2d455143bcb0f0d7bd46de6e06"
    }
  }
}

结果应该是这样的

["Platinum","Bronze","Gold"]

如何用Mongo聚合管道实现？ 这个问题有点像MongoDB：特定子文档的所有字段的聚合值，而答案中的Map-Reduce框架现已被弃用。

Answer 1

一个选项是：

db.collection.aggregate([
  {$project: {data: {$objectToArray: "$tier_and_details"}}},
  {$project: {_id: 0, data: "$data.v.tier"}}
])

查看它在 playground 示例中的工作原理

另一种选择是：

db.collection.aggregate([
  {$project: {
      _id: 0,
      data: {$map: {
          input: {$objectToArray: "$tier_and_details"},
          in: "$$this.v.tier"
      }}
  }}
])

查看它在 playground 示例中的工作原理