db={"member":[{_id:1,name:"great grandpa",children:[2]},{_id:2,name:"grandpa",children:[3]},{_id:3,name:"dad",children:[4,5]},{_id:4,name:"son"},{_id:5,name:"son2"}]}

db.member.aggregate([{$match:{"_id":1}},{"$graphLookup":{"from":"member","startWith":"$_id","connectFromField":"children","connectToField":"_id","as":"children"}}])