Mongodb聚合

Question

我遇到了mongoDB中的聚合问题。我正在处理的数据结构是这样的： -

{
        "_id" : ObjectId("4f16fe11d1e2d32371072aa0"),
        "body" : " \nHi Kate, per our discussion on yesterday about the $15.00 f
lat fee on Tom's \nand Mark's deals, here is Bloomberg's response.  Please pass
this info to all \nof our traders.  Please let me know what the response is from
 them.\n\nThanks\n\n\n---------------------- Forwarded by Evelyn Metoyer/Corp/En
ron on 04/17/2001 \n02:34 PM ---------------------------\n\n\n\"PAUL CALLAHAN, B
LOOMBERG/ NEW YORK\" <PCALLAHAN2@bloomberg.net> on 04/17/2001 \n02:28:57 PM\nTo:
 Evelyn.Metoyer@enron.com\ncc:  \n\nSubject: Commission\n\n\nEvelyn, as of April
 16, 2001 our charge for Spot trades is a flat fee of\n$15/trade.\n\n\n",
        "filename" : "3272.",
        "headers" : {
                "Content-Transfer-Encoding" : "7bit",
                "Content-Type" : "text/plain; charset=us-ascii",
                "Date" : ISODate("2001-04-17T14:33:00Z"),
                "From" : "evelyn.metoyer@enron.com",
                "Message-ID" : "<33504483.1075841847839.JavaMail.evans@thyme>",
                "Mime-Version" : "1.0",
                "Subject" : "Commission for Bloomberg",
                "To" : [
                        "kate.symes@enron.com"
                ],
                "X-FileName" : "kate symes 6-27-02.nsf",
                "X-Folder" : "\\kate symes 6-27-02\\Notes Folders\\Discussion th
reads",
                "X-From" : "Evelyn Metoyer",
                "X-Origin" : "SYMES-K",
                "X-To" : "Kate Symes",
                "X-bcc" : "",
                "X-cc" : ""
        },
        "mailbox" : "symes-k",
        "subFolder" : "discussion_threads"
}

数据库中有120477条记录。我应该找出那些倾向于彼此沟通最多（第二）的人。我写的查询如下：

db.messages.aggregate([{$project:{From:"$headers.From",To:"$headers.To",_id:0}
},{$unwind:"$headers.To"},{$group:{_id:{From:"$From",To:"$To"},number:{$sum:1}}}
,{$limit:3},{$sort:{number:-1}}]);

但它在某种程度上不起作用。