db={"xmantree":[{"_id":1,"name":"Dev"},{"_id":2,"name":"Eliot","parentId":1},{"_id":3,"name":"Ron","parentId":2},{"_id":4,"name":"Andrew","parentId":2},{"_id":5,"name":"Asya","parentId":3},{"_id":6,"name":"Dan","parentId":4}]}

db.xmantree.aggregate([{"$match":{name:"Eliot"}},{"$lookup":{"from":"xmantree",let:{parentId:"$_id"},pipeline:[{$match:{$expr:{$eq:["$$parentId","$parentId"]}}},{"$lookup":{"from":"xmantree","localField":"_id","foreignField":"parentId","as":"children"}}],"as":"children"}}])