MongoTemplate aggregate - group by date

I'm trying to create an aggregate query using mongotemplate where there's a grouping by date (i.e 2016-03-01) instead of datetime (i.e 2016-03-01 16:40:12). The dateToString operation exists in the mongodb documentation it can be used to extract the date from the datetime using formatting: https://docs.mongodb.org/manual/reference/operator/aggregation/dateToString/ but I get get it to work with mongotemplate - I get a NullPointerException. (my db version is 3.2)

List<AggregationOperation> aggregationOperations = new ArrayList<AggregationOperation>();

aggregationOperations.add(
          Aggregation.project("blabla", ...).
          andExpression("dateToString('%Y-%m-%d',timeCreated).as("date"));

aggregationOperations.add(Aggregation.group("date").sum("blabla").as("blabla"));

AggregationResults<?> aggregationResults = this.mongoTemplate.aggregate(
                        Aggregation.newAggregation(aggregationOperations),
                                    collectionName,
                                    resultClass);

When I use dayOfMonth(timeCreated) to extract the day, there's no exception, but I couldn't find and example of how to make this work with dateToString. I tried without '' for the date format, and it also didn't work...

This is the exception I get:

java.lang.NullPointerException
    at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:226)
    at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:194)
    at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:255)
    at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:194)
    at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:255)
    at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:194)
    at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:255)
    at org.bson.BasicBSONEncoder.putIterable(BasicBSONEncoder.java:324)
    at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:263)
    at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:194)
    at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:136)
    at com.mongodb.DefaultDBEncoder.writeObject(DefaultDBEncoder.java:36)
    at com.mongodb.OutMessage.putObject(OutMessage.java:289)
    at com.mongodb.OutMessage.writeQuery(OutMessage.java:211)
    at com.mongodb.OutMessage.query(OutMessage.java:86)
    at com.mongodb.DBCollectionImpl.find(DBCollectionImpl.java:81)
    at com.mongodb.DB.command(DB.java:320)
    at com.mongodb.DB.command(DB.java:299)
    at com.mongodb.DB.command(DB.java:374)
    at com.mongodb.DB.command(DB.java:246)
    at org.springframework.data.mongodb.core.MongoTemplate$2.doInDB(MongoTemplate.java:357)
    at org.springframework.data.mongodb.core.MongoTemplate$2.doInDB(MongoTemplate.java:355)
    at org.springframework.data.mongodb.core.MongoTemplate.execute(MongoTemplate.java:442)
    at org.springframework.data.mongodb.core.MongoTemplate.executeCommand(MongoTemplate.java:355)
    at org.springframework.data.mongodb.core.MongoTemplate.aggregate(MongoTemplate.java:1497)
    at org.springframework.data.mongodb.core.MongoTemplate.aggregate(MongoTemplate.java:1432)

EDIT:

Eventually we decided here on a different solution than what was suggested below, I'm writing it here in case anyone else finds it useful:

In addition to the "timeCreated" field which holds the datetime, we saved another field in the document: "date", that holds just the date (as long).

For example if "timeCreated" = "2015-12-24 16:36:06.657+02:00", then date is "2015-12-24 00:00:00", and we save 1449180000000. Now we can simply group by "date".

回答1:

You could try projecting the fields first by using the SpEL andExpression in the projection operation and then group by the new fields in the group operation:

Aggregation agg = newAggregation(
    project()       
        .andExpression("year(timeCreated)").as("year")
        .andExpression("month(timeCreated)").as("month")
        .andExpression("dayOfMonth(timeCreated)").as("day"),
    group(fields().and("year").and("month").and("day"))     
        .sum("blabla").as("blabla")
);

AggregationResults<BlaBlaModel> result = 
    mongoTemplate.aggregate(agg, collectionName, BlaBlaModel.class);
List<BlaBlaModel> resultList = result.getMappedResults();

回答2:

You could try using the DateOperators.DateToString class

aggregationOperations.add(
      Aggregation.project("blabla", ...).
      and(DateOperators.DateToString.dateOf("timeCreated").toString("%Y-%m-%d"));