Golang mgo returning value with aggregate $group

How do I get an aggregate query to return field values used in the $group statement.

The code:

type TheGroup struct{
	Id bson.ObjectId `json:"id,omitempty" bson:"_id,omitempty"` 
	Totalamount int
	Dayofyear int
	Actualyear string
	Transactiondate string
	Count int
}

var results []TheGroup

o1 := bson.M{"$match" :bson.M{"transactiontype": transactiontype},}
o2 := bson.M{"$group" : bson.M{"_id": bson.M{"day": "$dayofyear", "year":"$actualyear"},"totalamount":bson.M{"$sum":"$qty"}, "count":bson.M{"$sum":1}},}
operations := []bson.M{o1, o2}
pipe := collection.Pipe(operations)
err1 := pipe.All(&results)

if err := json.NewEncoder(w).Encode(results); err != nil {
panic(err)
}

The output is as follows:

[{"Totalamount":2061,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":679},{"Totalamount":8705,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":2145},{"Totalamount":8156,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":2806},{"Totalamount":9865,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3294},{"Totalamount":9619,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3102},{"Totalamount":9975,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3457},{"Totalamount":14839,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":4036},{"Totalamount":5100,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":1699},{"Totalamount":9649,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":2854},{"Totalamount":11457,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3220},{"Totalamount":12643,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3860},{"Totalamount":10301,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3620},{"Totalamount":7681,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":2816},{"Totalamount":8130,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3023}]

I think I understand why Dayofyear / Actualyear are not populated - because as there is no aggregration of the document values as the matched documents are traversed by the $group - but how do I get them to populate?

Running the following pipeline in mongo shell should give you the correct result:

pipeline = [
	{
		"$match": { "transactiontype": transactiontype }
	},
	{ 
		"$group": {
			"_id": {
				"year": { "$year": "$transactiondate" },
				"dayOfYear": { "$dayOfYear": "$transactiondate" }				
			},
		    "totalamount": { "$sum": "$qty" }, 
			"count": { "$sum": 1 },
			"date": { "$first": "$transactiondate"}
		}
	},
	{
		"$project": {
			"_id": 0,
			"totalamount": 1,
			"dayOfYear": "$_id.dayOfYear",
			"actualyear": { "$substr": [ "$_id.year", 0, 4 ] },
			"transactiondate": { 
				"$dateToString": { "format": "%Y-%m-%d %H:%M", "date": "$date" } 
			},
			"count": 1
		}
	}
]
db.collection.aggregate(pipeline)

of which the equivalent mGo expression follows (untested):

pipeline := []bson.M{	
	bson.M{
		"$match": bson.M{ "transactiontype": transactiontype }
	},
	bson.M{
		"$group": bson.M{
			"_id": bson.M{
				"year": bson.M{ "$year": "$transactiondate" },
				"dayOfYear": bson.M{ "$dayOfYear": "$transactiondate" }				
			},
		    "totalamount": bson.M{ "$sum": "$qty" }, 
			"count": bson.M{ "$sum": 1 },
			"date": bson.M{ "$first": "$transactiondate"}
		}
	},
	bson.M{
		"$project": bson.M{
			"_id": 0,
			"totalamount": 1,
			"dayOfYear": "$_id.dayOfYear",
			"actualyear": bson.M{ "$substr": []interface{}{ "$_id.year", 0, 4 } },
			"transactiondate": bson.M{ 
				"$dateToString": bson.M{ "format": "%Y-%m-%d %H:%M", "date": "$date" } 
			},
			"count": 1
		}
	}	
}

pipe := collection.Pipe(pipeline)