如何将对象数组转换为mongodb聚合中的对象

Question

我问过this question here，我得到了satisfactory answer。但是我有一个我无法解决的新问题。

我从query 得到的输出是一个对象数组。我怎样才能把这个数组变成一个对象？我已经知道how to do it in plain javascript，但是如何在 mongodb 聚合查询中做到这一点？

您可以假设您在聚合查询中有以下数据。

[
    {  "choiceA": [
            {"_id": ObjectId("..."), "voter": "Juzi", "choice": 0, "pollId": 100 },
            {"_id": ObjectId("..."), "voter": "Juma", "choice": 0, "pollId": 100 },
            {"_id": ObjectId("..."), "voter": "Jane", "choice": 0, "pollId": 100 },
    ]},
    {  "choiceB": [
            {"_id": ObjectId("..."), "voter": "Jamo", "choice": 1, "pollId": 100 },
            {"_id": ObjectId("..."), "voter": "Juju", "choice": 1, "pollId": 100 },
            {"_id": ObjectId("..."), "voter": "Jana", "choice": 1, "pollId": 100 }
    ]},
    {  "choiceC": [ ] }
]

如何将其变成如下所示的对象

{
    "choiceA": [
            {"_id": ObjectId("..."), "voter": "Juzi", "choice": 0, "pollId": 100 },
            {"_id": ObjectId("..."), "voter": "Juma", "choice": 0, "pollId": 100 },
            {"_id": ObjectId("..."), "voter": "Jane", "choice": 0, "pollId": 100 },
    ],
    "choiceB": [
            {"_id": ObjectId("..."), "voter": "Jamo", "choice": 1, "pollId": 100 },
            {"_id": ObjectId("..."), "voter": "Juju", "choice": 1, "pollId": 100 },
            {"_id": ObjectId("..."), "voter": "Jana", "choice": 1, "pollId": 100 }
    ],
    "choiceC": [ ] 
}

或者您可以直接回答this question，并将上面的结果作为输出。谢谢。

Answer 1

将这些步骤添加到您的管道中：

db.collection.aggregate([
  {
    $group: {
      _id: null,
      x: {
        $push: "$$ROOT"
      }
    }
  },
  {
    $project: {
      _id: 0,
      x: {
        $mergeObjects: "$x"
      }
    }
  },
  {
    $replaceRoot: {
      newRoot: "$x"
    }
  }
])

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