SERVER-8033 Sharded agg should only send needed fields from shards to mongos

*** BEFORE ***

> db.foo.aggregate([{$sort: {score:1}}, {$limit: 10}, {$project: {_id: 1}}], {explain:true})
{
    "splitPipeline" : {
        "shardsPart" : [
            {
                "$sort" : {
                    "sortKey" : {
                        "score" : 1
                    },
                    "limit" : NumberLong(10)
                }
            }
        ],
        "mergerPart" : [
            {
                "$sort" : {
                    "sortKey" : {
                        "score" : 1
                    },
                    "mergePresorted" : true,
                    "limit" : NumberLong(10)
                }
            },
            {
                "$project" : {
                    "_id" : true
                }
            }
        ]
    },
    "shards" : {
        "shard0000" : {
            "host" : "localhost:30001",
            "stages" : [
                {
                    "$cursor" : {
                        "query" : {

                        },
                        "indexOnly" : false,
                        "cursorType" : "BasicCursor"
                    }
                },
                {
                    "$sort" : {
                        "sortKey" : {
                            "score" : 1
                        },
                        "limit" : NumberLong(10)
                    }
                }
            ]
        }
    },
    "ok" : 1
}

*** AFTER ***

> db.foo.aggregate([{$sort: {score:1}}, {$limit: 10}, {$project: {_id: 1}}], {explain:true})
{
    "splitPipeline" : {
        "shardsPart" : [
            {
                "$sort" : {
                    "sortKey" : {
                        "score" : 1
                    },
                    "limit" : NumberLong(10)
                }
            },
            {
                "$project" : {
                    "score" : true,
                    "_id" : true
                }
            }
        ],
        "mergerPart" : [
            {
                "$sort" : {
                    "sortKey" : {
                        "score" : 1
                    },
                    "mergePresorted" : true,
                    "limit" : NumberLong(10)
                }
            },
            {
                "$project" : {
                    "_id" : true
                }
            }
        ]
    },
    "shards" : {
        "shard0000" : {
            "host" : "localhost:30001",
            "stages" : [
                {
                    "$cursor" : {
                        "query" : {

                        },
                        "fields" : {
                            "score" : 1,
                            "_id" : 1
                        },
                        "indexOnly" : false,
                        "cursorType" : "BasicCursor"
                    }
                },
                {
                    "$sort" : {
                        "sortKey" : {
                            "score" : 1
                        },
                        "limit" : NumberLong(10)
                    }
                },
                {
                    "$project" : {
                        "score" : true,
                        "_id" : true
                    }
                }
            ]
        }
    },
    "ok" : 1
}

1 files changed, 1 insertions, 1 deletions

diff --git a/src/mongo/db/pipeline/document_source_project.cpp b/src/mongo/db/pipeline/document_source_project.cpp
index 8ef3e1c6433..fdf9acc5b16 100644
--- a/src/mongo/db/pipeline/document_source_project.cpp
+++ b/src/mongo/db/pipeline/document_source_project.cpp
@@ -127,7 +127,7 @@ namespace mongo {
         pProject->_variables.reset(new Variables(idGenerator.getIdCount()));
 
         BSONObj projectObj = elem.Obj();
-        pProject->_raw = projectObj.getOwned(); // probably not necessary, but better to be safe
+        pProject->_raw = projectObj.getOwned();
 
 #if defined(_DEBUG)
         if (exprObj->isSimple()) {