php - mongo数据下的update问题

数据结构如下
array:1 [▼
  "59423208bff372441400002c" => array:30 [▼
    "_id" => MongoId {#687 ▶}
    "id" => 8850
    "author_id" => 83
    "category_id" => "10"
    "title" => "更多好看的看看 v 卡口就好吧"
    "view_type" => 1
    "post_type" => 1
    "body" => ""
    "tag" => ""
    "excerpt" => ""
    "lat" => 39.9457107
    "lng" => 116.3677488
    "sp" => array:2 [▶]
    "geo_encode" => "wx4g0pbkth4tbuu5ujz0g"
    "is_original" => 0
    "thumbnail" => []
    "platform" => 1
    "status" => 1
    "province" => "北京市"
    "city" => ""
    "district" => "朝阳区"
    "address" => "西直门宾馆"
    "featured" => 0
    "easemob_chatroom_id" => "18903238508541"
    "topic_length" => 24
    "ended_at" => "2017-06-15 15:36:55"
    "deleted_at" => "2017-06-15 15:36:55"
    "created_at" => "2017-06-14 15:34:40"
    "updated_at" => "2017-06-14 15:36:55"
    "comment" => array:2 [▼
      0 => array:12 [▼
        "id" => 64
        "easemob_chatroom_id" => "18903238508541"
        "message" => "23"
        "message_type" => "txt"
        "message_ext" => ""
        "uid" => 80
        "avatar" => "201705/080105491e54R7.jpg"
        "status" => 1
        "lng" => null
        "lat" => null
        "created_at" => "2017-06-12 11:04:27"
        "updated_at" => "2017-06-15 15:07:34"
      ]
      1 => array:12 [▼
        "id" => 61
        "easemob_chatroom_id" => "18903238508541"
        "message" => "4"
        "message_type" => "txt"
        "message_ext" => ""
        "uid" => 80
        "avatar" => "201705/080105491e54R7.jpg"
        "status" => 1
        "lng" => null
        "lat" => null
        "created_at" => "2017-06-12 11:04:27"
        "updated_at" => "2017-06-15 15:07:49"
      ]
    ]
  ]
]

问题：

1. 我使用了$push 往mongo里数据里压入了一个 comment[0]

2. 由于些原因，我不想要coment里某个值了，我改使用哪个操作删除任意的一个数组元素（就本题数组元素也为数组）

3. Mongodb数据更新命令 尝试了些方法，但是未生效，求指点！