Mongoose Group和Populate :获取一个人曾经与之交谈过的用户的唯一列表，比如whatsapp屏幕，在这里您有所有以前的用户。

Question

我一直在开发一个聊天应用程序，我需要显示与用户对话的用户列表。更像whatsapp的第一个屏幕，其中有所有发短信的用户的列表。

我的messagesSchema如下：

from : {type : mongoose.Types.ObjectId, ref : "Users"},
to : {type : mongoose.Types.ObjectId, ref : "Users"},
messageType : String,
messageContent :String,
images : [Object],
poster : Object,
status : String,
date: { type: Date, default: Date.now }

我正在使用nodejs，我尝试过很多农耕、group和其他猫鼬查询，但都失败了。

请帮助我查询，以获得所有不同的独特用户，一个人一直在与他们的最后一条消息交谈，并填充' to‘和’从‘。

Answer 1

数据：

[
  {
    "_id": ObjectId("100000000000000000000000"),
    "from": ObjectId("100000000000000000000000"),
    "to": ObjectId("200000000000000000000000"),
    "date": ISODate("2021-09-03T11:23:25.184Z"),
    "messageContent": "111"
  },
  {
    "_id": ObjectId("200000000000000000000000"),
    "from": ObjectId("200000000000000000000000"),
    "to": ObjectId("100000000000000000000000"),
    "date": ISODate("2021-09-02T11:23:25.184Z"),
    "messageContent": "222"
  },
  {
    "_id": ObjectId("300000000000000000000000"),
    "from": ObjectId("300000000000000000000000"),
    "to": ObjectId("100000000000000000000000"),
    "date": ISODate("2021-09-04T11:23:25.184Z"),
    "messageContent": "333"
  },
  {
    "_id": ObjectId("400000000000000000000000"),
    "from": ObjectId("300000000000000000000000"),
    "to": ObjectId("400000000000000000000000"),
    "date": ISODate("2021-09-05T11:23:25.184Z"),
    "messageContent": "444"
  }
]

聚合：(您是ObjectId("100000000000000000000000"))

db.collection.aggregate([
  {
    $match: {
      "$or": [
        { "from": ObjectId("100000000000000000000000") },
        { "to": ObjectId("100000000000000000000000") }
      ]
    }
  },
  {
    $project: {
      whom: {
        $first: {
          $filter: {
            input: [ "$from", "$to" ],
            as: "i",
            cond: { $ne: [ "$$i", ObjectId("100000000000000000000000") ]
            }
          }
        }
      },
      date: 1,
      messageContent: 1,
      to: 1,
      from: 1
    }
  },
  {
    $sort: { "date": -1 }
  },
  {
    $group: {
      _id: "$whom",
      date: { "$first": "$date" },
      message: { "$first": "$messageContent" },
      to: { "$first": "$to" },
      from: { "$first": "$from" }
    }
  }
])

结果：

[
  {
    "_id": ObjectId("300000000000000000000000"),
    "date": ISODate("2021-09-04T11:23:25.184Z"),
    "message": "333"
  },
  {
    "_id": ObjectId("200000000000000000000000"),
    "date": ISODate("2021-09-03T11:23:25.184Z"),
    "message": "222"
  }
]

示例： 蒙古操场