[{"key":1,"parentId":"dsdsdsdsdsds"},{"key":2,"parentId":"fdfdfdfdfd"}]

db.collection.aggregate([{"$addFields":{"parentIdCopy":{$cond:{if:{$ne:["$parentId",null]},then:{"$setField":{"field":{$literal:"$ref"},"input":{},"value":"parents"},},else:null}}}},{"$addFields":{"parentIdCopy":{$cond:{if:{$ne:["$parentId",null]},then:{"$setField":{"field":{$literal:"$id"},"input":"$parentIdCopy","value":"$parentId"},},else:null}}}},{"$set":{"parentId":"$parentIdCopy",parentIdCopy:"$$REMOVE"}},{"$merge":{"into":"collection","on":"_id","whenMatched":"replace",}}])