蒙戈聚合。 将匹配多个条件的数组元素作为新字段返回。 （包括复制）

Question

当前查询： https : //mongoplayground.net/p/mCD3vLoGG1q

上下文：用户对建议投赞成票或反对票。 建议和投票都在他们自己的收藏中。 我正在使用$lookup来获得一个建议的所有投票，它在聚合过程中变成了一个数组。

我想要每个建议的总票数，还想要 2 个特定用户的投票数据。

1. 登录的用户
2. 广播公司用户。

这个“投票数据”可以是一个简单的布尔值（用户是否投票），或者数组元素本身（首选）。 我只需要知道他们如何对建议进行投票。

当前结果

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    // WRONG. The broadcaster downvoted.
    "broadcasterUpvoted": true, 
    "hasUpvoted": true,
    "id": "sid",
    "votesLength": 2
  }
]

想要的结果

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
     // CORRECT!!!
    "broadcasterUpvoted": false,
    "hasUpvoted": true,
    "id": "sid",
    "votesLength": 2
  }
]

获取数组元素可能很有用，所以这个结果也很好。

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "broadcasterVote": {
      "suggestionId": "sid",
      "voteType": "downVote",
      "user": {
        "id": "broadcasterUserId"
      }
    },,
    "loggedInUserVote": {
      "suggestionId": "sid",
      "voteType": "upVote",
      "user": {
        "id": "loggedInUser"
      }
    },
    "id": "sid",
    "votesLength": 2
  }
]

编辑：另一个问题是，我如何使votesLength = (totalUpVotes - totalDownVotes)而不仅仅是sizeOfVotesArray

Answer 1

试试这个：

db.suggestions.aggregate([
  {
    $match: {
      id: "sid"
    }
  },
  {
    $lookup: {
      from: "votes",
      localField: "id",
      foreignField: "suggestionId",
      as: "votes"
    }
  },
  {
    $addFields: {
      hasUpvoted: {
        $filter: {
          input: "$votes",
          cond: { $eq: [ "apiCallUserId", "$$this.user.id"] }
        }
      },
      broadcasterUpvoted: {
        $filter: {
          input: "$votes",
          cond: { $eq: [ "broadcasterUserId", "$$this.user.id"]}
        }
      }
    }
  },
  {
    $project: {
      _id: 1,
      id: 1,
      votesLength: {
        $reduce: {
          input: "$votes",
          initialValue: 0,
          in: {
            $add: [
              "$$value",
              {
                $switch: {
                  branches: [
                    {
                      case: { $eq: ["$$this.voteType","upVote"]},
                      then: 1
                    },
                    {
                      case: { $eq: ["$$this.voteType", "downVote"]},
                      then: -1
                    }
                  ],
                  default: 0
                }
              }
            ]
          }
        }
      },
      broadcasterUpvoted: {
        $arrayElemAt: [
          "$broadcasterUpvoted",
          0
        ]
      },
      hasUpvoted: {
        $arrayElemAt: [
          "$hasUpvoted",
          0
        ]
      }
    }
  }
])

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