mongodb - 如何在 $lookup foreignField 上做一个子字符串

Question

db.tempCollection.aggregate(    
  {
    $lookup: { 
      "from" : "SensorInfo", 
      "localField" : "pid", 
      "foreignField" :{ $substr: [ "$pid", 0, 8 ] }, //Substring
      "as" : "sind_docs"
    }
  });

当我尝试使用上述语法时，看到 $ cannot be in field 错误消息。使用 pid 而不是 $pid 的类似消息

在这种情况下如何做 substr？

Answer 1

从 Mongo 3.6

db.tempCollection.aggregate(    
{
  $lookup: { 
    "from" : "SensorInfo",
    "let": { localPid: "$pid" },
    "pipeline": [
      { $match:
        { $expr:
          {
            $eq: [ "$$localPid",  { $substr: [ "$foreignPid", 0, 7 ] } ]
          }
        }
      },
    ],
    "as" : "sind_docs"
  }
});