Change Yii CActiveRecord column attribute value in case of storage array

Question

I have CActiveRecord model Game which attribute game_data stores the data of the game. In such way I stored the data (within beforeSave method):

$this->data = gzcompress(json_encode($this->data));

I extract data in the following way(withinn afterFind method):

$this->data = json_decode(gzuncompress($this->data), true);

After that I want to manipulate with the game_data attribute(add/change/delete), but the code returned an unexpected result for me. Code and my research:

var_dump($this->data); // data that already stored in db for this game 
array(1) {
  ["provably_fair"]=>
  array(1) {
    ["server_seed"]=>
    string(32) "dKOsfTsGvmvZe0VW6bcFnIhJKXDesKeM"
  }
} 

I am trying to add new property and value:

$this->data['provably_fair']['client_seed'] = $clientSeed;

But content of $this->data not change:

var_dump($this->data);
array(1) {
  ["provably_fair"]=>
  array(1) {
    ["server_seed"]=>
    string(32) "dKOsfTsGvmvZe0VW6bcFnIhJKXDesKeM"
  }
} 

How I can modify data of this CActiveRecord attribute and whether it is possible?