[{"id":"tweet_id","text":"this is the tweet's text","user":{"id":"user_id","name":"user_name",}},{"id":"tweet_id2","text":"this is the tweet's text","user":{"id":"user_id2","name":"user_name2",}}]

db.collection.aggregate([{"$group":{"_id":"$user.id","name":{"$first":"$user.name"}}},{"$replaceRoot":{"newRoot":{"$arrayToObject":[[{"k":"$_id","v":"$name"}]]}}}])