[{"_id":1,"name":"Bob","age":22},{"_id":2,"name":"Anna","age":30},{"_id":3,"name":"John","age":30},{"_id":4,"name":"Maria","age":22}]

db.collection.aggregate([{"$group":{"_id":"$age","field":{"$push":"$$ROOT"}}},{"$group":{"_id":"test","theFinalArray":{"$push":"$field"}}}])