Disjoint sets in Mongodb

Question

Hi have a MongoDB collection matchedpairs with a data structure as follows:

each document defines a pairwise connection with each other, i.e 1 is in union with 2 and 2 is in union with 10 etc. There are a large number of relationships defined.

{

    x:1,

    y:2

},

{

    x:2,

    y:10

},

{

    x:9,

    y:10

},

{

    x:8,

    y:4

}

I would like to query the documents and retrieve the unique disjoint sets for the pairs, i.e. return a result like this

{

    set:[1,2,9,10]

},

{

    set:[8,4]

}

I am familiar with the aggregation framework, but cannot see how to create the correct accumulator in the $group stage to create the disjoint sets. The attempt below simply gives just one grouping of similar pairs. As I see it I would have to create a whole string of $group stages (depending upon my set of data) to get the result I am looking for. Any clever ideas here?

db.matchedpairs.aggregate([

    {

        '$group': {

            '_id': '$y', 

            'like': {

                '$addToSet': '$x'

            }, 

            'from': {

                '$addToSet': '$y'

            }

        }

    }, {

        '$project': {

            '_id': 0, 

            'set': {

                '$setUnion': [

                    '$like', '$from'

                ]

            }

        }

    }

]

gives:

{

 set:[4,8]

},

{

 set:[10,2,9]

},

{

 set:[1,2]

}

Answer 1

It would be better to convert it into an array and map reduce can be use

db.matchedpairs.aggregate([

{ $project:{'set':['$x','$y']}},

{

        '$group': {

            '_id': '1', 

            'list': {

                '$addToSet': '$set'

            }

        }

    },

])

//gives

{

    "_id" : "1",

    "list" : [ 

        [ 

            1, 

            2

        ], 

        [ 

            9, 

            10

        ], 

        [ 

            2, 

            10

        ], 

        [ 

            8, 

            4

        ]

    ]

}