title: How to speed up performance of Django Rest Framework serializer with source from Model's method?
body: <p>I have API endpoint based on DRF 3.6.3 which already responsive (&lt;1s response time). But after I add new serializer field with source from model's method, it perform very slow (more than 30s response time).</p>
  
  <p><strong>edit:</strong> so the performance problem because the field that I process on the method is Foreign Key.</p>
  
  <p>this is my snippet code:</p>
  
  <pre><code>class Product(models.Model):
      a_name = models.CharField()
      b_name = models.ForeignKey(to=OutletProduct)
  
      def all_name(self):
          return u'%s %s' % (self.a_name, self.b_name)
  </code></pre>
  
  <p>Serializer:</p>
  
  <pre><code>class ProductSerializer(serializers.ModelSerializer):
      productid = serializers.ReadOnlyField(source='id')
      productallname = serializers.CharField(source='all_name')
  
      class Meta:
          model = Product
          fields = ('productid', 'productallname')
          read_only_fields = ('productallname', )
  </code></pre>
  
  <p>I only need to do small customization of my API output, and I don't think override <code>to_representation()</code> is right approach for this.</p>
  
  <p>thank you for your help.</p>