Mongo DB Aggregates - mongodb

Question

I'm stuck in a problem of aggregates in mongoDB. The data structure that I'm dealing with is like this :-
{
"_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$15/trade.\n\n\n",
"filename" : "3272.",
"headers" : {
"Content-Transfer-Encoding" : "7bit",
"Content-Type" : "text/plain; charset=us-ascii",
"Date" : ISODate("2001-04-17T14:33:00Z"),
"From" : "evelyn.metoyer#enron.com",
"Message-ID" : "<33504483.1075841847839.JavaMail.evans#thyme>",
"Mime-Version" : "1.0",
"Subject" : "Commission for Bloomberg",
"To" : [
"kate.symes#enron.com"
],
"X-FileName" : "kate symes 6-27-02.nsf",
"X-Folder" : "\\kate symes 6-27-02\\Notes Folders\\Discussion th
reads",
"X-From" : "Evelyn Metoyer",
"X-Origin" : "SYMES-K",
"X-To" : "Kate Symes",
"X-bcc" : "",
"X-cc" : ""
},
"mailbox" : "symes-k",
"subFolder" : "discussion_threads"
}
There are 120477 records in the database. I'm supposed to find out the pair of people who tend to communicate most (and 2nd most) with each other. The query that I've written is as follows:
db.messages.aggregate([{$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}}]);
but it somehow does not work.