MongoDb: Aggregation using Groovy language

Question

Iam using groovy scripting under SpagoBI. I want to use aggregation. I want for example to execute the following aggregation:

db.myCollection.aggregate(
   [
      {
        $group : {
           _id : { day: { $dayOfMonth: "$recvTime" } }

        }
      }
   ]
)

I tried:

DBObject projectFields = new BasicDBObject('$dayOfMonth',"recvTime");
DBObject project=new BasicDBObject('$project',projectFields)

DBObject groupFields = new BasicDBObject( "_id",project);

DBObject group = new BasicDBObject('$group', groupFields);

iterable = db.getCollection('myCollection').aggregate(group)  

I got this error:

An unexpected error occured while executing dataset: { "serverUsed" : "192.168.1.160:27017" , "errmsg" : "exception: invalid operator '$project'" , "code" : 15999 , "ok" : 0.0}

Any ideas?

Updates: the query executed in Mongo shell

db['cygnus_/kurapath_enocean_power_enocean'].aggregate(
...    [
...       {
...         $group : {
...            _id : { day: { $dayOfMonth: "$recvTime" } }
...         }
...       }
...    ]
... );
{ "_id" : { "day" : 9 } }
{ "_id" : { "day" : 8 } }
{ "_id" : { "day" : 7 } }
{ "_id" : { "day" : 4 } }
{ "_id" : { "day" : 3 } }

the data stored in mongo db:

db['cygnus_/kurapath_enocean_power_enocean'].find()
{ "_id" : ObjectId("55e81e9631d7791085668331"), "recvTime" : ISODate("2015-09-03T10:19:02Z"), "attrName" : "power", "attrType" : "string", "attrValue" : "2085.0" }
{ "_id" : ObjectId("55e81e9631d7791085668332"), "recvTime" : ISODate("2015-09-03T10:19:02Z"), "attrName" : "power", "attrType" : "string", "attrValue" : "2085.0" }
{ "_id" : ObjectId("55e81e9831d7791085668333"), "recvTime" : ISODate("2015-09-03T10:19:04Z"), "attrName" : "power", "attrType" : "string", "attrValue" : "2077.0" }

Answer 1

From the error, the aggregation is not expecting the $project operator so you should change the projectFields and project variables to show the actual pipeline expressions i.e.

DBObject dateFields = new BasicDBObject("$dayOfMonth", "$recvTime");
DBObject dateObject = new BasicDBObject("day", dateFields);

DBObject groupFields = new BasicDBObject( "_id", dateObject);
DBObject group = new BasicDBObject('$group', groupFields);

iterable = db.getCollection('myCollection').aggregate(group);