Bade's Problem

README.md

The Question

Collection contains a pair of strings Obj.a and Obj.b, find out the sum of all unique pairs?

Note Obj{a:"abc",b:"xyz"} is equivalent to Obj{a:"xyz",b:"abc"}, although they are two separate documents.

aggregation.js

db.bade.aggregate([
    {$group: { _id: "$_id", a: { $push: "$a" }, b: { $push: "$b"}, c: { $first: "$c" }}},
    {$project: { d: { $setUnion: ["$a", "$b"]}, c: 1 } },
    {$unwind: "$d"},
    {$sort: { d: 1}},
    {$group: { _id: "$_id", d: { $push: "$d"}, c: { $max: "$c" } }},
    {$group: { _id: "$d", c: { $sum: "$c"}}}
]);

answer.txt

{ "_id" : [ "efg", "xyz" ], "c" : 30 }
{ "_id" : [ "abc", "tuv" ], "c" : 10 }
{ "_id" : [ "abc", "xyz" ], "c" : 15 }
{ "_id" : [ "hij", "xyz" ], "c" : 9 }
{ "_id" : [ "abc", "hij" ], "c" : 15 }
{ "_id" : [ "klm", "xyz" ], "c" : 1 }
{ "_id" : [ "opq", "xyz" ], "c" : 20 }

data.js

var aaas = [{a:"abc",b:"xyz",c:10},{a:"xyz",b:"abc",c:5},{a:"efg",b:"xyz",c:10},{a:"xyz",b:"efg",c:20},{a:"abc",b:"tuv",c:0},{a:"tuv",b:"abc",c:10},{a:"abc",b:"hij",c:15},{a:"hij",b:"xyz",c:9},{a:"klm",b:"xyz",c:1},{a:"opq",b:"xyz",c:10},{a:"xyz",b:"opq",c:10}];
for (i in aaas) {
    db.bade.save(aaas[i]);
}