title: Django Rest Framework override model fields in modelserialzer
body: <p>Let's say I have this serializer:</p>
  
  <pre><code>class DashboardItemSerializer(ModelSerializer):
      offer = serializers.SerializerMethodField()
      cart_item = serializers.SerializerMethodField()
      stock_details = serializers.SerializerMethodField()
  
      class Meta:
          model = OrderItem
          fields = ('uuid', 'seller', 'total', 'tax', 'offer', 'shipping_address', 'cart_item', 'stock_details')
  
      def offer(self, obj):
          return 123
  
      def cart_item(self, obj):
          return 123
  
      def stock_details(self, obj):
          return 123
  </code></pre>
  
  <p>these fields <code>offer</code>, <code>cart_item</code> and <code>stock_details</code> are model fields, and I would like to override it with to return different values, but looks like DRF ignores the customized fields and return the original value in obj.</p>
  
  <p>If I don't put them in the <code>fields</code> list, the data will simply not include those fields.</p>
  
  <pre><code>class DashboardItemSerializer(ModelSerializer):
      offer = serializers.SerializerMethodField()
      cart_item = serializers.SerializerMethodField()
      stock_details = serializers.SerializerMethodField()
  
      class Meta:
          model = OrderItem
          fields = ('uuid', 'seller', 'total', 'tax', 'shipping_address')
  
      def offer(self, obj):
          return 123
  
      def cart_item(self, obj):
          return 123
  
      def stock_details(self, obj):
          return 123
  </code></pre>
  
  <p>One way to do it is to <a href="http://www.django-rest-framework.org/api-guide/serializers/#specifying-fields-explicitly" rel="noreferrer">define fields explicitly</a>, but I wonder if there is a easier way.</p>