use extractFields which is now correct to fix SERVER-257

db/dbcommands.cpp

@@ -1161,19 +1161,7 @@ namespace mongo {
         }
 
         BSONObj getKey( const BSONObj& obj , const BSONObj& keyPattern , const string keyFunction , double avgSize ){
-            BSONObjBuilder b( (int)(avgSize * 1.1) );
-
-            BSONObjIterator i( keyPattern );
-            while ( i.more() ){
-                const char * n = i.next().fieldName();
-                BSONElement e = obj[n];
-                if ( e.eoo() )
-                    b.appendNull( n );
-                else
-                    b.append( e );
-            }
-
-            return b.obj();
+            return obj.extractFields( keyPattern );
         }
         
         bool group( string realdbname , auto_ptr<DBClientCursor> cursor , BSONObj keyPattern , string keyFunction , string reduceCode , BSONObj initial , string& errmsg , BSONObjBuilder& result ){

jstests/group1.js

@@ -38,3 +38,23 @@ c = {key: {a:1}, cond: {}, initial: {"count": 0}, reduce: function(obj, prev) {
 assert.eq( t.group( c ) , t.groupcmd( c ) , "ZZZZ" );
 
 
+t.drop();
+
+t.save( { name : { first : "a" , last : "A" } } );
+t.save( { name : { first : "b" , last : "B" } } );
+t.save( { name : { first : "a" , last : "A" } } );
+
+
+p = { key : { 'name.first' : true } , 
+      reduce : function(obj,prev) { prev.count++; },
+      initial: { count: 0 }
+    };
+
+res = t.group( p );
+assert.eq( 2 , res.length , "Z1" );
+assert.eq( "a" , res[0].first , "Z2" )
+assert.eq( "b" , res[1].first , "Z3" )
+assert.eq( 2 , res[0].count , "Z4" )
+assert.eq( 1 , res[1].count , "Z5" )
+
+