[{"_id":"11111","parent_id":"99","name":"AAAA"},{"_id":"11112","parent_id":"99","name":"BBBB"},{"_id":"11113","parent_id":"100","name":"CCCC"},{"_id":"11114","parent_id":"99","name":"DDDD"}]

db.collection.aggregate([{"$group":{"_id":"$parent_id","item":{"$push":"$name"},"data":{"$push":{"_id":"$_id","name":"$name"}}}},{"$unwind":"$data"},{"$project":{"_id":"$data._id","parent_id":"$_id","name":"$data.name","item":1}}])