title: Golang mgo returning value with aggregate $group
body: <p>How do I get an aggregate query to return field values used in the $group statement.</p>
  
  <p>The code:</p>
  
  <pre><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(&amp;results)
  
  if err := json.NewEncoder(w).Encode(results); err != nil {
  panic(err)
  }
  </code></pre>
  
  <p>The output is as follows:</p>
  
  <pre><code>[{"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}]
  </code></pre>
  
  <p>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? </p>