MongoDB - joining two results of queries and dense_rank - mongodb

Question

I'm learning about MongoDB and I have some problems with understanding its concept.
I have a collection which looks like that:
db.email.findOne()
{
"_id" : ObjectId("52af48b5d55148fa0c199646"),
"sender" : "tori.wells#enron.com",
"recipients" : [
"michael#optsevents.com"
],
"cc" : [ ],
"text" : "Mr. Christman:\n\nThank you for your invitation for Dr. Lay to speak at your upcoming forum in \nFrance, the format looks wonderful. Unfortunately, Dr. Lay has calendar \nconflicts and will be unable to participate.\n\nIf you should need further assistance, please do not hesitate to contact us.\n\nTori Wells\nExecutive Assistant",
"mid" : "22263156.1075840285610.JavaMail.evans#thyme",
"fpath" : "enron_mail_20110402/maildir/lay-k/_sent/101.",
"bcc" : [ ],
"to" : [
"michael#optsevents.com"
],
"replyto" : null,
"ctype" : "text/plain; charset=us-ascii",
"fname" : "101.",
"date" : "2000-08-04 09:04:00-07:00",
"folder" : "_sent",
"subject" : "Wall Street Journal Millennium Forum"
}
It's the Enron database.
I'm trying to make a query which will return listed emails with amount of messages sent by it and amount of messages received by it.
I managed to make two queries which looks like that:
db.email.aggregate({$group:{_id:"$sender",SendsAmount:{$sum:1}}},{$sort:{SendsAmount:-1}})
{ "_id" : "rosalee.fleming#enron.com", "SendsAmount" : 849 }
{ "_id" : "brown_mary_jo#lilly.com", "SendsAmount" : 82 }
{ "_id" : "leonardo.pacheco#enron.com", "SendsAmount" : 78 }
db.email.aggregate({$group:{_id:"$recipients",ReceivedAmount:{$sum:1}}},{$unwind:"$_id"},{$sort:{ReceivedAmount:-1}})
{ "_id" : "klay#enron.com", "ReceivedAmount" : 1350 }
{ "_id" : "kenneth.lay#enron.com", "ReceivedAmount" : 912 }
{ "_id" : "kenneth.lay#enron.com", "ReceivedAmount" : 78 }
As you can see first one returns me emails and amount of emails sends from it and second one also returns emails and amount of received ones by it.
My point is to join(?) these two into one and get one query which will return something like:
{ "_id" : "email#enron.com", "SendsAmount" : 57, "ReceivedAmount": 43 }
I know there is $lookup but it can be used only with two collections, so my idea was to make two collections out of these two queries but I'm feeling like there is better way of solving my problem.
---My second problem is about trying to do some DENSE_RANK which is not present in MongoDB. I want to rank email adresses by amount of sent emails.
I used the $unwind and insertArrayIndex but I got something like ROW_NUMBER which is not I'm looking for.
I have written something like that:
db.email.aggregate({$group:{"_id":"$sender",SendsAmount:{$sum:1},rank:0}},{$sort:{"ile":-1}}).forEach(function(x){
var howmany=0;
var query=db.email.aggregate({$group:{"_id":"$sender",SendsAmount:{$sum:1}}},{$match:{ile:{$gt:x.SendsAmount}}},{$group:{_id:null, HowManyGreater:{$sum:1}}});
query.forEach(function(y){
howmany=y.HowManyGreater;
});
howmany=howmany+1;
print("email: "+ x._id + " SendsAmount: " + x.SendsAmount + " rank " + howmany+1);
});
Which is givig me the result I want, but it's not even the document but only printed information. I've read about MapReduce but I didn't get the idea how to use it in this case.

Answer 1

If you want to do all calculations in aggregate query, you can use $facet and $group stages as below
db.email.aggregate([
{
$facet: {
send: [
{
$group: {
_id: "$sender",
SendsAmount: {
$sum: 1
}
}
},
{
$sort: {
SendsAmount: -1
}
}
],
recieve: [
{
$group: {
_id: "$recipients",
ReceivedAmount: {
$sum: 1
}
}
},
{
$unwind: "$_id"
},
{
$sort: {
ReceivedAmount: -1
}
}
]
}
},
{
$project: {
all: {
$concatArrays: [
"$recieve",
"$send"
]
}
}
},
{
$unwind: "$all"
},
{
$group: {
_id: "$all._id",
ReceivedAmount: {
$sum: {
$cond: {
if: {
$gt: [
"$all.ReceivedAmount",
null
]
},
then: "$all.ReceivedAmount",
else: 0
}
}
},
SendsAmount: {
$sum: {
$cond: {
if: {
$gt: [
"$all.SendsAmount",
null
]
},
then: "$all.SendsAmount",
else: 0
}
}
}
}
}
])