title: django rest framework nested relationships
body: <p>In my model</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>class Province(models.Model):
      name = models.CharField(max_length=128, verbose_name=u"province")
  
  class City(models.Model):
      province = models.ForeignKey("system.Province", related_name='cities', verbose_name=u"province")
      name = models.CharField(max_length=128, verbose_name=u"name")
  
  class District(models.Model):
      city = models.ForeignKey("system.City", related_name='districts', verbose_name=u"city")
      name = models.CharField(max_length=128, verbose_name=u"name")
  </code></pre>
  
  <hr>
  
  <p><strong>serializers.py</strong></p>
  
  <pre><code>class DistrictSerializer(serializers.ModelSerializer):
      class Meta:
          model = District
          fields = ('id', 'name')
  
  
  class CitySerializer(serializers.ModelSerializer):
      districts = DistrictSerializer(many=True, read_only=True)
  
      class Meta:
          model = City
          fields = ('id', 'name', 'districts')
  
  
  class ProvinceSerializer(serializers.ModelSerializer):
      cities = CitySerializer(many=True, read_only=True)
  
      class Meta:
          model = Province
          fields = ('id', 'name', 'cities')
  </code></pre>
  
  <hr>
  
  <p>When i make <strong>GET</strong> request to the url <strong>/system/province/1/</strong>, i am getting json string like</p>
  
  <pre><code>{
  "id": 1,
  "name": "广东省",
  "cities": [
      {
          "id": 1,
          "name": "广州市",
          "districts": [
              {
                  "id": 1,
                  "name": "天河区"
              },
              {
                  "id": 2,
                  "name": "海珠区"
              }
          ]
      },
      {
          "id": 2,
          "name": "汕头市",
          "districts": [
              {
                  "id": 3,
                  "name": "朝阳区"
              }
          ]
      },
      {
          "id": 3,
          "name": "湛江市",
          "districts": []
      },
      {
          "id": 4,
          "name": "韶关市",
          "districts": []
      }
  ]
  </code></pre>
  
  <p>}</p>
  
  <p>the districts json is not what i want。
  how can i prevent auto load districts</p>