How to separate array from nested arrays with MongoDB and PHP aggregate?

I would like to get the distinct code_y values, and I use below code, the output is equal to select distinct code_y from table_x,

$res = $db_x->command (
        array(
            "aggregate" => "table_x",
            "pipeline" =>
                array(
                    array( '$group' => array( "_id" => ['id' =>'$code_y', 'name' => '$code_x', 'color' => '$color']))),
            "cursor" => ['batchSize' => 200]
        )
);

I got results as below, I don’t need [_id] => Array, just id, name and color, How to separate array from nested arrays with MongoDB and PHP aggregate? Or how to put [_id] => Array together with id, name and color? as in my front-end htmls pages, I got [object] [Object] because of those nested arrays.

---

   [0] => Array
        (
            [_id] => Array
                (
                    [id] => a1
                    [name] => bbb
                    [color] => blue
                )

        )

    [1] => Array
        (
            [_id] => Array
                (
                    [id] => a2
                    [name] => aaa
                    [color] => blue
                )

        )

    [2] => Array
        (
            [_id] => Array
                (
                    [id] => a3
                    [name] => abc
                    [color] =>red
                )

        )

---

What I want is as below:

[0] => Array
    (
       [id] => a1
       [name] => bbb
       [color] => blue
    )

[1] => Array
    (
       [id] => a2
       [name] => aaa
       [color] => blue


    )

[2] => Array
    (
      [id] => a3
      [name] => abc
      [color] =>red
    )

Answer

What you need is a another stage after the group.

You could either use project and list each field:

array( '$project' => array( 
     "id" => '$_id.id',
     "name" => '$_id.name',
     "color" => '$_id.color'
))

or $replaceRoot to grab them all:

array( '$replaceRoot' => array( "newRoot" => '$_id' ))