How to convert dates in an array with $dateFromString?

Question

I'm trying to convert timestamps in an array to dates with $dateFromString

Sample document which I'm trying to convert dates from:

{
    "_id" : ObjectId("5cbc5efc8af5053fd8bdca31"),
    "ticker" : "ticker",
    "currency" : "currency",
    "daily" : [ 
        {
            "timestamp" : "2019-04-18",
            "open" : "5.3300",
            "high" : "5.3300",
            "low" : "5.2000",
            "close" : "5.2700",
            "volume" : "6001"
        }, 
        {
            "timestamp" : "2019-04-17",
            "open" : "5.1500",
            "high" : "5.2900",
            "low" : "5.1500",
            "close" : "5.2700",
            "volume" : "37659"
        }, 
        {
            "timestamp" : "2019-04-16",
            "open" : "4.7100",
            "high" : "5.3000",
            "low" : "4.7100",
            "close" : "5.1500",
            "volume" : "112100"
        }
    ]
}

Aggregation query in pymongo:

db.test.aggregate([{
        '$project': {
            'daily.timestamp': {
                '$dateFromString': {
                    'dateString': '$daily.timestamp',
                    'format':  '%Y-%m-%d'
                }
            }
        }
    }])

This throws the following error:

pymongo.errors.OperationFailure: $dateFromString requires that 'dateString' be a string, found: array with value ["2019-04-18", "2019-04-17", "2019-04-16", "2019-04-15"....]

Is it even possible to apply $dateFromString to an array with hundreds of elements?

sheilak · Accepted Answer · 2019-05-11 19:08:29Z

1

You can use the $map aggregation operator to apply $dateFromString to each element in the array:

db.test.aggregate([{
  "$project": {
    "ticker": 1,
    "currency": 1,
    "daily": {
      "$map": {
        "input": "$daily",
        "in": {
          "timestamp": { 
            "$dateFromString": {
              "dateString": '$$this.timestamp',
              "format":  '%Y-%m-%d'
            }
          },
          "open": "$$this.open",
          "high": "$$this.high",
          "low": "$$this.low",
          "close": "$$this.close",
          "volume": "$$this.volume"
        }
      }
    }
  }
}])

edited May 11, 2019 at 19:08

answered May 11, 2019 at 13:30

sheilak

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