mongodb - 如何在$ lookup foreignField

Question

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

当我尝试使用上面的语法时，看到$不能出现在字段错误消息中。使用pid而不是$ pid的类似消息

在这种情况下如何做substr？

Answer 1

来自 Mongo 3.6

SELECT [FirstName], [Email]

FROM [t1]

WHERE [Email] like '%' || [FirstName]  || '%'