Mongodb recursive query

Question

I have the following schema in my taxon collection :

{ 
  \"_id\": 1, 
  \"na\": [ \"root_1\",
        \"root_2\",
        \"root_3\" ], 
  \"pa\":         


        

Answer 1

You can try below aggregation.

Stages $match - $graphLookup - $project.

$reduce to pick the first element from the each of $graphLookup nameList's na array.

db.taxon.aggregate([{
    $match: {
        _id: {
            $in: listId
        }
    }
}, {
    $graphLookup: {
        from: "taxon",
        startWith: "$_id",
        connectFromField: "pa",
        connectToField: "_id",
        as: "nameList"
    }
}, {
    $project: {
        nameList: {
            $reduce: {
                input: "$nameList",
                initialValue: [],
                in: {
                    "$concatArrays": ["$$value", {
                        $slice: ["$$this.na", 1]
                    }]
                }
            }
        }
    }
}])