MongoDB mapreduce return null value

The following works for me, indicating that your map-reduce command is fine as described. Please try the following, exactly as written and see if you get the same result. If you do, then your problem is elsewhere, not anything described in your question. Hope that this helps.

orders7.js

db.ORDERS7.drop();
db.query.drop();
doc0 = {
        "O_ORDERKEY" : NumberLong(359),
        "O_CUSTKEY" : {
                "$ref" : "CUSTOMER",
                "$id" : ObjectId("4f973ff37d6517e9723c4d63")
        },
        "O_ORDERSTATUS" : "F",
        "O_TOTALPRICE" : 239998.53,
        "O_ORDERDATE" : ISODate("1994-12-19T02:00:00Z"),
        "O_ORDERPRIORITY" : "3-MEDIUM",
        "O_CLERK" : "Clerk#000000934",
        "O_SHIPPRIORITY" : 0,
        "O_COMMENT" : "furiously final foxes are. regular,"
};
doc1 = {
        "O_ORDERKEY" : NumberLong(359),
        "O_CUSTKEY" : {
                "$ref" : "CUSTOMER",
                "$id" : ObjectId("4f973ff37d6517e9723c4d63")
        },
        "O_ORDERSTATUS" : "F",
        "O_TOTALPRICE" : 239998.53,
        "O_ORDERDATE" : ISODate("1994-12-19T02:00:00Z"),
        "O_ORDERPRIORITY" : "3-MEDIUM",
        "O_CLERK" : "Clerk#000000934",
        "O_SHIPPRIORITY" : 0,
        "O_COMMENT" : "furiously final foxes are. regular,"
};
db.ORDERS7.save( doc0 );
db.ORDERS7.save( doc1 );
printjson(db.ORDERS7.find().toArray());

printjson(db.runCommand({
    mapreduce: "ORDERS7",
    query: {
        O_SHIPPRIORITY: 0
    },
    map : function Map() {
        emit("sum",this.O_TOTALPRICE);
    },
    reduce : function Reduce(key, values) {
        var sum = 0;
        for (var i = 0; i < values.length; i++) {
            sum += values[i];
        }
        return sum;
    },
    out: 'query'
}));
printjson(db.query.find().toArray());

$ mongo xyzzy orders7.js

MongoDB shell version: 2.1.2
connecting to: xyzzy
[
    {
        "_id" : ObjectId("4ffa368b9ee7cfbc46a3990a"),
        "O_ORDERKEY" : NumberLong(359),
        "O_CUSTKEY" : DBRef("CUSTOMER", ObjectId("4f973ff37d6517e9723c4d63")),
        "O_ORDERSTATUS" : "F",
        "O_TOTALPRICE" : 239998.53,
        "O_ORDERDATE" : ISODate("1994-12-19T02:00:00Z"),
        "O_ORDERPRIORITY" : "3-MEDIUM",
        "O_CLERK" : "Clerk#000000934",
        "O_SHIPPRIORITY" : 0,
        "O_COMMENT" : "furiously final foxes are. regular,"
    },
    {
        "_id" : ObjectId("4ffa368b9ee7cfbc46a3990b"),
        "O_ORDERKEY" : NumberLong(359),
        "O_CUSTKEY" : DBRef("CUSTOMER", ObjectId("4f973ff37d6517e9723c4d63")),
        "O_ORDERSTATUS" : "F",
        "O_TOTALPRICE" : 239998.53,
        "O_ORDERDATE" : ISODate("1994-12-19T02:00:00Z"),
        "O_ORDERPRIORITY" : "3-MEDIUM",
        "O_CLERK" : "Clerk#000000934",
        "O_SHIPPRIORITY" : 0,
        "O_COMMENT" : "furiously final foxes are. regular,"
    }
]
{
    "result" : "query",
    "timeMillis" : 22,
    "counts" : {
        "input" : 2,
        "emit" : 2,
        "reduce" : 1,
        "output" : 1
    },
    "ok" : 1
}
[ { "_id" : "sum", "value" : 479997.06 } ]