[{"_id":{"$oid":"6364f2eee35fc06fa06afb5f"},"type":"subbranch","parent_id":"635116c18fe4294398842ebb","org_name":"Pune - Delhi"},{"_id":{"$oid":"635116c18fe4294398842ebb"},"type":"branch","org_name":"Delhi Branch","parent_id":"0"}]

db.collection.aggregate([{$addFields:{"parent_id":{$convert:{input:"$parent_id",to:"objectId",onError:"$parent_id",}}}},{$graphLookup:{from:"collection",startWith:"$parent_id",connectFromField:"parent_id",connectToField:"_id",as:"parent"}}])