我有3种文件如下所列：

vilya_be> db.plots.find({})
[
  {
    _id: ObjectId("6426b069ca83da130cdb7f70"),
    x: 0,
    y: 0,
    _class: 'com.vilya.farm.domain.model.Plot'
  }
]

vilya_be> db.users_have_plots.find({})
[
  {
    _id: ObjectId("6426b073ca83da130cdb7f71"),
    userId: '6412c76956d4170a7de34d92',
    plot: DBRef("plots", ObjectId("6426b069ca83da130cdb7f70")),
    _class: 'com.vilya.farm.domain.model.UserPlot'
  }
]

vilya_be> db.users.find({})
[
  {
    _id: ObjectId("6412c76956d4170a7de34d92"),
    email: 'abc@abc.com',
    password: '$2a$10$s9VgOYd.fOKZF66TnAsjWemiCYkA7aG45NJpuSNgbVxpcIGF7fWqu',
    firstName: 'f',
    lastName: 'l',
    plots: [ DBRef("users_have_plots", ObjectId("6426b073ca83da130cdb7f71")) ],
    _class: 'com.vilya.farm.domain.model.User'
  },
  {
    _id: ObjectId("6414667360e4ba4481052627"),
    email: 'abc1@abc.com',
    password: '$2a$10$OP52phZ61l2JX2e2TQOu9ubYFBYcPeqEZ92ox2Nyyp5e.MEZk7GhS',
    firstName: 'f',
    lastName: 'l',
    _class: 'com.vilya.farm.domain.model.User'
  }
]

我想用一个命令加载所有的users沿着users_have_plots（以及plots）。
我试过：

db.users.aggregate([
{
  "$lookup": {
    "from": "users_have_plots",
    "let": {
      "plots": "$plots"
    },
    "pipeline": [
      {
        "$match": {
          "$expr": {
            "$in": [
              "$_id",
              "$$plots"
            ]
          }
        }
      },
      {
        "$lookup": {
          "from": "plots",
          "localField": "plot",
          "foreignField": "_id",
          "as": "plot"
        }
      }
    ],
    "as": "plots"
  }
}
]);

它给了我：PlanExecutor error during aggregation :: caused by :: $in requires an array as a second argument, found: missing
还有这个

db.users.aggregate([
{
  "$lookup": {
    "from": "users_have_plots",
    "let": {
      "plots": "$plots"
    },
    "pipeline": [
      {
        "$match": {
          "$expr": {
            "$in": [
              "$_id",
              "$$plots.ObjectId"
            ]
          }
        }
      },
      {
        "$lookup": {
          "from": "plots",
          "localField": "plot",
          "foreignField": "_id",
          "as": "plot"
        }
      }
    ],
    "as": "plots"
  }
}
]);

它也给了我：PlanExecutor error during aggregation :: caused by :: $in requires an array as a second argument, found: missing
我是mongodb的新手，只是找不到任何方法来使它工作。任何帮助都将不胜感激！
mongo版本：mongo：6.0.2-focal在docker桌面上运行
编辑：尝试：

db.users.aggregate([{
  "$lookup": {
    "from": "users_have_plots",
    "let": {
      "plots": "$plots"
    },
    "pipeline": [
      {
        "$match": {
          "$expr": {
            "$in": [
              "$_id",
              {
                "$ifNull": [
                  "$$plots.ObjectId",
                  []
                ]
              }
            ]
          }
        }
      },
      {
        "$lookup": {
          "from": "plots",
          "localField": "plot",
          "foreignField": "_id",
          "as": "plot"
        }
      }
    ],
    "as": "plots"
  }
}
])

给我：

[
  {
    _id: ObjectId("6412c76956d4170a7de34d92"),
    email: 'abc@abc.com',
    password: '$2a$10$s9VgOYd.fOKZF66TnAsjWemiCYkA7aG45NJpuSNgbVxpcIGF7fWqu',
    firstName: 'f',
    lastName: 'l',
    plots: [],
    _class: 'com.vilya.farm.domain.model.User'
  },
  {
    _id: ObjectId("6414667360e4ba4481052627"),
    email: 'abc1@abc.com',
    password: '$2a$10$OP52phZ61l2JX2e2TQOu9ubYFBYcPeqEZ92ox2Nyyp5e.MEZk7GhS',
    firstName: 'f',
    lastName: 'l',
    _class: 'com.vilya.farm.domain.model.User',
    plots: []
  }
]

未包括地块。