The "out" option in the MapReduce command must be ordered. This was

found after the implementation was accepting maps for a long time,
so rather than breaking the API, the implementation will fix the
order if necessary.

Details about the order requirement may be seen in MongoDB's code:

    http://goo.gl/L8jwJX

Thanks to Peter Kleiweg for reporting the issue.

LICENSE

@@ -1,6 +1,6 @@
 mgo - MongoDB driver for Go
 
-Copyright (c) 2010-2012 - Gustavo Niemeyer <[email protected]>
+Copyright (c) 2010-2013 - Gustavo Niemeyer <[email protected]>
 
 All rights reserved.
 

session.go

@@ -2841,7 +2841,7 @@ func (q *Query) MapReduce(job *MapReduce, result interface{}) (info *MapReduceIn
 		Map:        job.Map,
 		Reduce:     job.Reduce,
 		Finalize:   job.Finalize,
-		Out:        job.Out,
+		Out:        fixMROut(job.Out),
 		Scope:      job.Scope,
 		Verbose:    job.Verbose,
 		Query:      op.query,
@@ -2895,6 +2895,36 @@ func (q *Query) MapReduce(job *MapReduce, result interface{}) (info *MapReduceIn
 	return info, nil
 }
 
+// The "out" option in the MapReduce command must be ordered. This was
+// found after the implementation was accepting maps for a long time,
+// so rather than breaking the API, we'll fix the order if necessary.
+// Details about the order requirement may be seen in MongoDB's code:
+//
+//     http://goo.gl/L8jwJX
+//
+func fixMROut(out interface{}) interface{} {
+	outv := reflect.ValueOf(out)
+	if outv.Kind() != reflect.Map || outv.Type().Key() != reflect.TypeOf("") {
+		return out
+	}
+	outs := make(bson.D, outv.Len())
+
+	outTypeIndex := -1
+	for i, k := range outv.MapKeys() {
+		ks := k.String()
+		outs[i].Name = ks
+		outs[i].Value = outv.MapIndex(k).Interface()
+		switch ks {
+		case "normal", "replace", "merge", "reduce", "inline":
+			outTypeIndex = i
+		}
+	}
+	if outTypeIndex > 0 {
+		outs[0], outs[outTypeIndex] = outs[outTypeIndex], outs[0]
+	}
+	return outs
+}
+
 type Change struct {
 	Update    interface{} // The change document
 	Upsert    bool        // Whether to insert in case the document isn't found

session_test.go

@@ -2850,6 +2850,29 @@ func (s *S) TestMapReduceToOtherDb(c *C) {
 	c.Assert(iter.Close(), IsNil)
 }
 
+func (s *S) TestMapReduceOutOfOrder(c *C) {
+	session, err := mgo.Dial("localhost:40001")
+	c.Assert(err, IsNil)
+	defer session.Close()
+
+	coll := session.DB("mydb").C("mycoll")
+
+	for _, i := range []int{1, 4, 6, 2, 2, 3, 4} {
+		coll.Insert(M{"n": i})
+	}
+
+	job := &mgo.MapReduce{
+		Map:    "function() { emit(this.n, 1); }",
+		Reduce: "function(key, values) { return Array.sum(values); }",
+		Out:    bson.M{"a": "a", "z": "z", "replace": "mr", "db": "otherdb", "b": "b", "y": "y"},
+	}
+
+	info, err := coll.Find(nil).MapReduce(job, nil)
+	c.Assert(err, IsNil)
+	c.Assert(info.Collection, Equals, "mr")
+	c.Assert(info.Database, Equals, "otherdb")
+}
+
 func (s *S) TestMapReduceScope(c *C) {
 	session, err := mgo.Dial("localhost:40001")
 	c.Assert(err, IsNil)