How can I write a MongoDB query to return an array of arrays

Question

Suppose that I have the following data in a collection called "users":

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

I want to write a query "grouping" the users by the age, so it will return an array of array. Something like this:

[
  [
    {
      "_id": 1,
      "name": "Bob",
      "class": 2
    },
    {
      "_id": 2,
      "name": "Anna",
      "class": 2
    }
  ],
  [
    {
      "_id": 3,
      "name": "John",
      "class": 5
    },
    {
      "_id": 4,
      "name": "Maria",
      "class": 5
    }
  ]
]

I've tried to write the query using aggregation, but the name is not an accumulator object, so it will never work.

db.users.aggregate([{"$group": {_id:"$age", _name:"$name"}}])

I've also read some stack overflow questions but I can't figure out how to solve my issue.

Answer 1

Maybe something like this:

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

Explained:

1. Group all documents by $age pushing the docs in array field
2. Group all documents in another array with all field arrays inside

Playground