MongoDB索引转换为numberlong

I'm trying to create a MongoDB index on a specific field with the following input:

Collection: messages

source_id (integer)
message_id (string)
subject (string)
timestamp (integer)

I'm in need to query this collection on the source and uid fields.

Example document:

{ 
 "_id" : ObjectId("53aad315bffebc5d378b4569"), 
 "source_id" : NumberLong(1), 
 "message_id" : "AMA9D2TP9zWA.kuhlman.lowell@dietrich.com", 
 "subject" : "Labore aut est amet temporibus mollitia et.", 
 "timestamp" : NumberLong(1364645191) 
}

A simple db.messages.getIndexes() returns that my message_id index is a NumberLong?

> db.messages.getIndexes()
[
{
    "v" : 1,
    "name" : "_id_",
    "key" : {
        "_id" : 1
    },
    "ns" : "hs.messages"
},
{
    "v" : 1,
    "name" : "s_1",
    "key" : {
        "s" : NumberLong(1)
    },
    "ns" : "hs.messages"
},
{
    "v" : 1,
    "name" : "m_1",
    "key" : {
        "m" : NumberLong(1)
    },
    "ns" : "hs.messages"
}
]