Group using the value from two possible fields - mongodb

Question

Let's say I have a match collection in the following format
{user1: "a", user2: "b"},
{user1: "a", user2: "c"},
{user1: "b", user2: "d"},
{user1: "b", user2: "c"},
{user1: "b", user2: "e"},
{user1: "c", user2: "f"}
I would like to know which user has the most appearance (either in user1 or user2). The result should be in this format ordered by the number of occurence.
{"user": "b", count:4},
{"user": "c", count:3},
{"user": "a", count:2},
{"user": "d", count:1},
{"user": "f", count:1},
{"user": "e", count:1}
Is there a way I can group on the value of two fields?
Something like match.aggregate({$group: {_id: {$or:["user1","user2]}}, count:{$sum:1}})

Answer 1

db.match.aggregate([
{$project: { user: [ "$user1", "$user2" ]}},
{$unwind: "$user"},
{$group: {_id: "$user", count: {$sum:1}}}
])
First stage projects each document into array of users
{user: ["a", "b"]},
{user: ["a", "c"]},
{user: ["b", "d"]},
...
Next we unwind arrays
{user:"a"},
{user:"b"},
{user:"a"},
{user:"c"},
{user:"b"},
...
And simple grouping at the end

Answer 2

Basically the concept is to $map onto an array and work from there:
db.collection.aggregate([
{ "$project": {
"_id": 0,
"user": { "$map": {
"input": ["A","B"],
"as": "el",
"in": {
"$cond": {
"if": { "$eq": [ "$$el", "A" ] },
"then": "$user1",
"else": "$user2"
}
}
}}
}},
{ "$unwind": "$user" },
{ "$group": {
"_id": "$user",
"count": { "$sum": 1 }
}}
])

Answer 3

Let us take an example and go through
db.users_data.find();
{
"_id" : 1,
"user1" : "a",
"user2" : "aa",
"status" : "NEW",
"createdDate" : ISODate("2016-05-03T08:52:32.434Z")
},
{
"_id" : 2,
"user1" : "a",
"user2" : "ab",
"status" : "NEW",
"createdDate" : ISODate("2016-05-03T09:52:32.434Z")
},
{
"_id" : 3,
"user1" : "b",
"user2" : "aa",
"status" : "NEW",
"createdDate" : ISODate("2016-05-03T10:52:32.434Z")
},
{
"_id" : 4,
"user1" : "b",
"user2" : "ab",
"status" : "NEW",
"createdDate" : ISODate("2016-05-03T10:52:32.434Z")
},
{
"_id" : 5,
"user1" : "a",
"user2" : "aa",
"status" : "OLD",
"createdDate" : ISODate("2015-05-03T08:52:32.434Z")
},
{
"_id" : 6,
"user1" : "a",
"user2" : "ab",
"status" : "OLD",
"createdDate" : ISODate("2015-05-03T08:52:32.434Z")
},
Then
db.users_data.aggregate([
{"$group" : {_id:{user1:"$user1",user2:"$user2"}, count:{$sum:1}}} ])
])
will give the resuls as
{ "_id" : { "user1" : "a", "user2" : "aa" }, "count" : 2}
{ "_id" : { "user1" : "a", "user2" : "ab" }, "count" : 2}
{ "_id" : { "user1" : "b", "user2" : "aa" }, "count" : 1}
{ "_id" : { "user1" : "b", "user2" : "ab" }, "count" : 1}
Thus grouping by multiple ids are possible
Now one more variation
db.users_data.aggregate([
{"$group" : {_id:{user1:"$user1",user2:"$user2",status:"$status"}, count:{$sum:1}}} ])
])
will give the resuls as
{ "_id" : { "user1" : "a", "user2" : "aa","status":"NEW" }, "count" : 1}
{ "_id" : { "user1" : "a", "user2" : "ab","status":"NEW" }, "count" : 1}
{ "_id" : { "user1" : "b", "user2" : "aa","status":"NEW" }, "count" : 1}
{ "_id" : { "user1" : "b", "user2" : "ab","status":"NEW" }, "count" : 1}
{ "_id" : { "user1" : "a", "user2" : "aa","status":"OLD" }, "count" : 1}
{ "_id" : { "user1" : "a", "user2" : "ab","status":"OLD" }, "count" : 1}