Mongo aggregation Convert string to date in version 3.4.7

问题

I have the following aggregator function, which I am not firing in mongo console directly. I am passing this aggregator to an API as a query string. I will encode the following aggregator directly and pass to a get request as a query string.

[

      {
         "$limit": 500
      },

      {
            "$match": {
              "timestamp": {
                "$gte": {
                  "$dte": "2017-07-01"
                },
                "$lt": {
                  "$dte": "2020-12-31"
                }
              }
            }
      },      

      {
            "$group": {
                "_id": { "$year": "$timestamp" },
                "scores": { "$push": "$statement.result.score.scaled" },
                "duration_seconds": { "$push": "$statement.result.duration_seconds" }
             }
      },

      { 
            "$project": {
                 "scores": 1,
                 "duration_seconds": 1,
                 "statements_count": 1,
                 "semester": { 
                                "$cond": [  
                                            { 
                                                "$lte": [ 
                                                          {  
                                                            "$month": 
                                                                { 
                                                                  "$dateFromString": {
                                                                    "dateString": "$timestamp"
                                                                  }
                                                                }
                                                          }, 6
                                                        ] 
                                            }, "Jan", "July" 
                                         ]
                             }
            }  
      },

      {
            "$group": {
                "_id": { "semester": "$semester", "year": { "$year": "$timestamp" } },
                "avg_score": { "$avg": "$scores" },
                "avg_duration": { "$avg": "$duration_seconds" },
                "statements_count": { "$sum": 1 }
             }
      }
    ]

Here I have a statement document, it has a timestamp that is a ISO date but in string format. I want to group the statements by semester (if month is 1-6: Jan, 6-12: July)

Here the problem is I am using mongo 3.4.7 and "$dateFromString" operator only works in above 3.6 versions. So I am getting an error.

1. So how can I group this by semester in mongo 3.4.7 version ?

2. I am facing the following issue while grouping by semester first.

   And my aggregation not works for grouping by semester. Because how can I use the semester grouping first and do the project stage? Group stage is not supporting $cond etc aggregator that I can use only in project stage.

回答1:

You can try $split operator with $arrayElemAt for index 1 to return month value.

{"semester":{
  "$cond":[
    {"$lte":[{"$arrayElemAt":[{"$split":["$timestamp","-"]},1]},"06"]},
    "Jan",
    "July"
  ]
}}

来源：https://stackoverflow.com/questions/51188337/mongo-aggregation-convert-string-to-date-in-version-3-4-7