Database
[{userid:"MongoDB",userpid:"Databases"},{userid:"dbm",userpid:"Databases"},{userid:"Databases",userpid:"Programming"},{userid:"Languages",userpid:"Programming"},{userid:"Programming",userpid:"Books"},{userid:"Books",userpid:null}]
Query
db.collection.aggregate([{$graphLookup:{from:"collection",startWith:"$userpid",connectFromField:"userpid",connectToField:"userid",as:"prerequisites",maxDepth:3}}])