Datetime to string in mongodb where only month and year are in data

Question

suppose this is my Data:-

[
  {"Id_Cust": "4145","firstName": "Albade","lastName": "Maazou", "gender": "female","date": "031981"},
  {"Id_Cust": "5296", "firstName": "Rafael","lastName": "Oliveira","gender": "male","date": "061987"},
  {"Id_Cust": "6192","firstName": "Abdul Rahman","lastName": "Budjana","gender": "male","date": "011990"}
]

I try by $datetostring and $datefromstring I also try format but still got an Error:- But, Nothing happened.

OperationFailure: PlanExecutor error during aggregation :: caused by :: an incomplete date/time string has been found, with elements missing: "031981", full error: {'ok': 0.0, 'errmsg': 'PlanExecutor error during aggregation :: caused by :: an incomplete date/time string has been found, with elements missing: "031981"', 'code': 241, 'codeName': 'ConversionFailure'}

Answer 1

Mongo requires the "date" part to fully exist, this means it expects to get at least a year, a month and a day.

So you can either pad your string with 01 and provide the format option:

db.collection.aggregate([
  {
    $addFields: {
      date: {
        "$dateFromString": {
          "dateString": {
            "$concat": [
              "01",
              "$date"
            ]
          },
          "format": "%d%m%Y"
        }
      }
    }
  }
])

Mongo Playground

Or use $dateFromParts instead:

db.collection.aggregate([
  {
    $addFields: {
      date: {
        "$dateFromParts": {
          year: {
            "$toInt": {
              "$substr": [
                "$date",
                2,
                4
              ]
            }
          },
          month: {
            $toInt: {
              $substr: [
                "$date",
                0,
                2
              ]
            }
          }
        }
      }
    }
  }
])

Mongo Playground

---

Bonus - This requirement is not stated explicitly in the documentation but can be found in the source code:

if (s->time->y != TIMELIB_UNSET && s->time->m != TIMELIB_UNSET &&
    s->time->d != TIMELIB_UNSET &&
    !timelib_valid_date( s->time->y, s->time->m, s->time->d)) {
    add_pbf_warning(s, TIMELIB_WARN_INVALID_DATE, "The parsed date was invalid", string, ptr);
}