我有一个MostRecentMessage模型，定义为

const MostRecentMessage = new Schema({
  to: {
    type: mongoose.Schema.Types.ObjectId,ref: "user"
  },from: {
    type: mongoose.Schema.Types.ObjectId,conversation: {
    type: mongoose.Schema.Types.ObjectId,ref: "conversation"
  },date: {
    type: Date,default: Date.now
  }
});

每当用户A向用户B发送消息时，都会创建一个文档（如果该文档不存在）。每次用户A向用户B发送消息时，都会使用新的conversation和date更新此文档。文件的示例是

_id: ObjectId(5dc46521cf670a0017d2434d)
to: ObjectId(5dc464ce2fd75700178c1ad4) // User B
from: ObjectId(5dc464fc2fd75700178c1ad5) // User A
conversation: ObjectId(5dc465c6cf670a0017d24363)
date: 2019-11-07T18:40:33.242+00:00
__v: 0

想法是，这将有助于跟踪所有最近发给特定用户的消息。

同样，用户B可以向用户A发送消息。与上面类似，将创建一个新文档，

_id: ObjectId(5dc46521cf670a0017d2434d)
to: ObjectId(5dc464fc2fd75700178c1ad5) // User A
from: ObjectId(5dc464ce2fd75700178c1ad4) // User B
conversation: ObjectId(5dc465c6cf670a0017d24363)
date: 2019-11-07T18:40:33.242+00:00
__v: 0

此文档将随着用户B发送给用户A的每条新消息而更新。

现在，下面是我的聚合查询..但问题是它只返回一个对话。可能有10个用户与用户A交谈，但仍只返回一个文档。

  const { id } = req.user;

  try {
    await MostRecentMessages.aggregate(
      [
        {
          $match: {
            $or: [
              { from: mongoose.Types.ObjectId(id) },{ to: mongoose.Types.ObjectId(id) }
            ]
          }
        },{ $project: { _id: 1,from: 1,to: 1,conversation: 1,date: 1 } },{ $sort: { date: -1 } },{
          $group: {
            _id: null,from: { $first: "$from" },to: { $first: "$to" },date: { $first: "$date" },conversation: { $first: "$conversation" }
          }
        },{
          $lookup: {
            from: "conversations",localField: "conversation",foreignField: "_id",as: "conversation"
          }
        },{ $unwind: { path: "$conversation" } },{
          $lookup: {
            from: "users",localField: "to",as: "to"
          }
        },{ $unwind: { path: "$to" } },localField: "from",as: "from"
          }
        },{ $unwind: { path: "$from" } }
      ],function(err,docs) {
        if (err) {
          console.log(err);
        } else {
          return res.json(docs);
        }
      }
    );
  } catch (err) {
    console.log(err);

    return res.status(500).send("Server error");
  }

我在这里做什么错了？

编辑：我现在所拥有的：

使用此代码，如果

User A messages User B,'Test1'
User B messages User A,'Test2'
User A messages User B,'Test3'

然后，两个用户都只会显示Test2消息。如何让“ Test3”同时显示给两个用户？

    await MostRecentMessages.aggregate(
      [
        {
          $match: {
            $or: [
              { from: mongoose.Types.ObjectId(id) },{ to: mongoose.Types.ObjectId(id) }
            ]
            // deletedBy: { $ne: id }
          }
        },{
          $group: {
            _id: {
              userConcerned: {
                $cond: {
                  if: {
                    $eq: ["$to",mongoose.Types.ObjectId(id)]
                  },then: "$to",else: "$from"
                }
              },interlocutor: {
                $cond: {
                  if: {
                    $eq: ["$to",then: "$from",else: "$to"
                }
              }
            },docs) {
        if (err) {
          console.log(err);
        } else {
          return res.json(docs);
        }
      }