// works
db.newTest.explain(1).aggregate([
   {$sort:{valueDate:1}},
   {$group:{_id: "$valueDate", first: {$first: "$farmId"}}}
]).stages[0]["$cursor"]["executionStats"]
{
        "executionSuccess" : true,
        "nReturned" : 2,
        "executionTimeMillis" : 0,
        "totalKeysExamined" : 2,
        "totalDocsExamined" : 2,
... }
// doesn't work
db.newTest.explain(1).aggregate([
  {$sort:{valueDate:1}},
  {$group:{_id: {v:"$valueDate"}, first: {$first: "$farmId"}}}
]).stages[0]["$cursor"]["executionStats"]
{
        "executionSuccess" : true,
        "nReturned" : 5,
        "executionTimeMillis" : 0,
        "totalKeysExamined" : 10,
        "totalDocsExamined" : 5,
... }