title: django rest frame work filter a serializer field
body: <p>i had django api and its serializer and models...
  i use filters like this:</p>
  
  <pre><code>class OrderFilter(django_filters.FilterSet):
  
  class Meta:
      model = Dealer
      fields = {
          'first_name': ['icontains', 'exact'],
          'last_name': ['icontains', 'exact'],
          'mobile': ['icontains', 'exact'],
          'is_active': ['exact'],
          'agent': ['exact'],
      }
  </code></pre>
  
  <p>all of this fields was django model field but in my serializer i have a SerializerMethodField like this:</p>
  
  <pre><code>class DealerListSerializer(serializers.ModelSerializer):
  
  online = serializers.SerializerMethodField()
  
  def get_online(self, obj):
      r = Redis.get_instance().conn
      key = settings.ONLINE_PREFIX.format(obj.id)
      if r.get(key):
          return True
      return False
  </code></pre>
  
  <p>i would like to apply filter on online field.</p>
  
  <pre><code>class OrderFilter(django_filters.FilterSet):
  online = django_filters.rest_framework.BooleanFilter(method='filter_online')
  
  def filter_online(self, queryset, name, value):
      ?????
  
  class Meta:
      model = Dealer
      fields = {
          'first_name': ['icontains', 'exact'],
          'last_name': ['icontains', 'exact'],
          'mobile': ['icontains', 'exact'],
          'is_active': ['exact'],
          'agent': ['exact'],
          'online': ['exact'],
      }
  </code></pre>
  
  <p>what should I do?
  thanks</p>