exception: can't convert from BSON type EOO to Date

Question

I am getting an issue for running the following aggregate query:

db.snippets.aggregate([ { \'$project\': { month: { \'$month\': \'$created_at\' }} } ])


        

Answer 1

I had a related issue, but in my case the Date fields were the members of an array, so the error was "can't convert BSON type Object to Date".

I needed to get the day of week from the dates in the possibleTripDateTimes array.

Sample document:

{
"possibleTripDateTimes" : [
    {
        "tripDateTime" : ISODate("2015-08-01T06:00:00.000-0700")
    }
]
}

The fix was simply to use dot notation to address the array member fields.

db.trips.aggregate([
  {
       $project: {
         departTime: {
           $map: {
             input: "$possibleTripDateTimes.tripDateTime",
             as: "dateTime",
             in: { $dayOfWeek: "$$dateTime" }
           }
   }
  }
}
]
);

I hope this helps someone who also gets zero search results on the "BSON type Object" search

Answer 2

First, you need to check whether the data type is in ISODate. IF not you can change the data type as the following example.

db.collectionName.find().forEach(function(each_object_from_collection){each_object_from_collection.your_date_field=new ISODate(each_object_from_collection.your_date_field);db.collectionName.save(each_object_from_collection);})

Now you can find it in two ways

db.collectionName.find({ $expr: {$eq: [{ $year: "$your_date_field" }, 2017]}});

Or by aggregation

db.collectionName.aggregate([{$project: {field1_you_need_in_result: 1,field12_you_need_in_result: 1,your_year_variable: {$year: '$your_date_field'}, your_month_variable: {$month: '$your_date_field'}}},{$match: {your_year_variable:2017, your_month_variable: 3}}])