Database
[{_id:"12345",model:"Honda"},{_id:"12346",model:"Honda"},{_id:"12347",model:"Honda"},{_id:"12348",model:"Toyota"},{_id:"12349",model:"Volkswagen"},{_id:"12350",model:"Volkswagen"},]
Query
db.collection.aggregate([{$group:{_id:"$model",doc:{$first:"$$ROOT"}}},{"$unwind":"$doc"},{"$replaceRoot":{"newRoot":"$doc"}}])