title: How can I only depth one field in Django-Rest-Framework?
body: <p>I have a <code>IPv4ListSerializer</code>, it has a <code>depth = 1</code>:</p>
  
  <pre><code>class IPv4ListSerializer(ModelSerializer):
      """
      ipv4
      """
      ip_status = serializers.CharField(read_only=True)
      class Meta:
          model = IPv4Manage
          fields = "__all__"
          depth = 1
  </code></pre>
  
  <p>but the result it become every field <code>depth 1</code>:</p>
  
  <pre><code>[
      {
          "id": 281,
          "ip_status": "Vlaned",
          "ip": "43.243.33.1",
          "netmask": "255.255.255.248",
          "prefix": 29,
          "is_gateway": false,
          "is_network_ip": false,
          "is_broadcast_ip": false,
          "desc": null,
          "ctime": "2018-04-26T21:17:33.623092+08:00",
          "uptime": "2018-06-02T12:48:41.882016+08:00",
          "ipv4network": {
              "id": 21,
              "network_ip": "43.243.33.0",
              "prefix": 24,
              "ctime": "2018-04-26T21:17:33.533667+08:00",
              "uptime": "2018-04-26T21:17:33.533814+08:00",
              "area_partition": 7
          },
          "vlaned_ipv4network": {
              "id": 1,
              "network_ip": "43.243.33.0",
              "prefix": 29,
              "gateway_ip": "43.243.33.6",
              "broadcast_ip": "43.243.33.7",
              "ctime": "2018-04-26T21:27:56.011624+08:00",
              "uptime": "2018-04-26T21:27:56.258053+08:00",
              "ipv4network": 21,
              "vlanedipv4networkgroup": 4
          },
          "physical_server": {
              "id": 23,
              "name": "SE01-A1",
              "desc": "desc",
              "cpu": "Intel Xeon E3-1230 v5",
              "ram": "Supermicro  DDR4___16",
              "disk": "Inter SSD___240",
              "price": "1000.00",
              "server_status": "ready",
              "pay_time": null,
              "expire_time": null,
              "ipmi_addr": "172.16.30.1",
              "ipmi_account_and_pwd": null,
              "has_intranet": true,
              "intranet_ip": null,
              "customer_desc": null,
              "whmcs_tblorders_id": null,
              "ctime": "2018-05-08T16:15:50.639962+08:00",
              "uptime": "2018-08-20T18:25:27.289100+08:00",
              "switches": 3,
              "physical_server_model": 5,
              "switchesport": 57,
              "cabinet": 3,
              "task": null,
              "user": null,
              "trade_record": null
          }
      },
      ....
  ]
  </code></pre>
  
  <p>Can I only depth one field? such as if <code>depth</code> <code>ipv4network</code>, my requirement is:</p>
  
  <pre><code>[
      {
          "id": 281,
          "ip_status": "Vlaned",
          "ip": "43.243.33.1",
          "netmask": "255.255.255.248",
          "prefix": 29,
          "is_gateway": false,
          "is_network_ip": false,
          "is_broadcast_ip": false,
          "desc": null,
          "ctime": "2018-04-26T21:17:33.623092+08:00",
          "uptime": "2018-06-02T12:48:41.882016+08:00",
          "ipv4network": {
              "id": 21,
              "network_ip": "43.243.33.0",
              "prefix": 24,
              "ctime": "2018-04-26T21:17:33.533667+08:00",
              "uptime": "2018-04-26T21:17:33.533814+08:00",
              "area_partition": 7
          },
          "vlaned_ipv4network": 1,
          "physical_server": 23,
      },
      ....
  ]
  </code></pre>