[{"_id":1,"name":"Alex","parentId":0},{"_id":2,"name":"Michael","parentId":1},{"_id":3,"name":"George","parentId":2},{"_id":4,"name":"Yuri","parentId":1}]

db.collection.aggregate([{/** start with somebody, or in this case* somebody's parentId*/"$match":{"parentId":2}},{/** go all the way up the parent tree*/"$graphLookup":{"from":"collection","startWith":"$parentId","connectFromField":"parentId","connectToField":"_id","depthField":"depth","as":"parents"}}])