Storing array as list doesn't work

Question

I'm trying to insert an array in my mongo collection like this

field:[tag1,tag2,tag3]

my mapping is

/**
 * @MongoDB\Hash
 */
protected $field=array();

I'm inserting like this in PHP :

$message->setField([$idme,$to]);

While I want it stored as list, I get this format:

 "field" : {
                "0" : "533f28c9211b6f7e448b4567",
                "1" : "52cb29b0211b6fd9248b456b"
        }

How can I achieve my goal ?