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  1. 1. 关联查询

关联查询

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db.task.aggregate([
  { $lookup: {
    let: { "modeId":{ "$toObjectId": "$modem" } },
    from: "modem",
    pipeline: [
      { "$match": { "$expr": { "$eq": [ "$_id", "$$modeId" ] } } }
    ],
    as: "output"
  }},
  {
    $set: {
      modem_name: { $arrayElemAt: ["$output.name", 0] }
    }
  },{
  $project: {
    output: 0
  }
}])