Mongodb counting combination of items in array

Question

I have something like: `

[
{
    ....
    tags : ["A","B"]
},
{
    ....
    tags : ["A","B"]
},
{
    ....
    tags : ["J","K"]
},
{
    ....
    tags : ["A","B","C"]
}
]`

With the Aggregation Framwork I'd like to group by array combinations to have something like this:

[
{
    _id:["A","B"],
    count : 3
},
{
    _id:["J","K"],
    count : 1
},
{
    _id:["A","C"],
    count : 1

   },
{
        _id:["B","C"],
        count : 1
    },
]

is it possible? Thanks in advance

Answer 1

Query

• for each tag of size 1, makes it ["V"] -> ["V" null] (if you dont want to count the pairs with the null you can filter out in the final results)
• map and for each member we say its together with the rest members For example ["A" "B" "C"] will make "A" together with ["B" "C"], "B" together with ["A" "C"] etc (we count double, we will divide next with 2)
• unwind that arrays
• group by pairs, but shorted pairs ["A" "B"] = ["B" "A"]
• take the count 0.5 (instead of 1) (its like we divide with 2)

Test code here

aggregate(
[ {
  "$set" : {
    "tags" : {
      "$cond" : [ {
        "$eq" : [ {
          "$size" : "$tags"
        }, 1 ]
      }, {
        "$concatArrays" : [ "$tags", [ null ] ]
      }, "$tags" ]
    }
  }
}, {
  "$set" : {
    "a" : {
      "$map" : {
        "input" : "$tags",
        "in" : {
          "member" : "$$t",
          "togetherWith" : {
            "$setDifference" : [ "$tags", [ "$$t" ] ]
          }
        },
        "as" : "t"
      }
    }
  }
}, {
  "$unwind" : {
    "path" : "$a"
  }
}, {
  "$replaceRoot" : {
    "newRoot" : "$a"
  }
}, {
  "$unwind" : {
    "path" : "$togetherWith"
  }
}, {
  "$group" : {
    "_id" : {
      "$cond" : [ {
        "$lt" : [ "$member", "$togetherWith" ]
      }, [ "$member", "$togetherWith" ], [ "$togetherWith", "$member" ] ]
    },
    "count" : {
      "$sum" : 0.5
    }
  }
}, {
  "$set" : {
    "count" : {
      "$toInt" : "$count"
    }
  }
} ]
)