[{"_id":1,"difficulty_level":1},{"_id":2,"difficulty_level":2},{"_id":3,"difficulty_level":3},{"_id":4,"difficulty_level":1},{"_id":5,"difficulty_level":2},{"_id":6,"difficulty_level":1},{"_id":7,"difficulty_level":3}]

db.collection.aggregate([{"$facet":{"difficulty_level_1":[{"$match":{"difficulty_level":1}},{"$sample":{"size":1}}],"difficulty_level_2":[{"$match":{"difficulty_level":2}},{"$sample":{"size":1}}]}}])