db={"user":[{"_id":1,"nombre":"n1"},{"_id":2,"nombre":"n2"},{"_id":3,"nombre":"n3"}],"trans":[{"user_id":1,"trans_tipo":"bank"},{"user_id":2,"trans_tipo":"bank"},{"user_id":3,"trans_tipo":"no"}]}

db.user.aggregate([{"$lookup":{"from":"trans","localField":"_id","foreignField":"user_id","as":"trans"}},{"$unwind":"$trans"},{"$match":{"trans.trans_tipo":"bank"}},{"$group":{"_id":"$_id","nombre":{"$first":"$nombre"},"trans":{"$push":"$trans"}}}])