title: How to call instance methods on the nested Serializer with django rest?
body: <p>I've got two models I'm trying to nest together. <code>Timesheet</code> and <code>Invoice</code></p>
  <p>My <code>InvoiceSerializer</code> looks something like this:</p>
  <pre><code>class InvoiceSerializer(serializers.ModelSerializer):
  
      billable_timesheets = serializers.SerializerMethodField() 
      total_hours_and_cost = serializers.SerializerMethodField() 
  
      class Meta:
          model = Invoice
          fields = (
              &quot;hours&quot;,
              &quot;hour_cost&quot;,
              &quot;billable_timesheets&quot;,
              &quot;total_hours_and_cost&quot;, 
              ...
  
              )
  
      def get_total_hours_and_cost(self, obj):
          return obj.hours * obj.hour_cost
  
      def get_billable_timesheets(self, obj):
          &quot;&quot;&quot;Getting all timesheets for selected billable period&quot;&quot;&quot;
          timesheets = obj.project.timesheets.filter(&lt;queryset here&gt;)
          return TimesheetSerializer(timesheets, many=True).data
  </code></pre>
  <p>This works fine and all - I can define MethodFields and the correct JSON is returned, great. However, I got a method on my child model (in this case, the <code>Timesheet</code> model) that I need to access and run some calculations on. I'm getting the data necessary via <code>get_billable_timesheets</code>, and now I need to run a method on my <code>Timesheet model</code> called <code>total_duration()</code>. Whenever I try to do something along the lines of</p>
  <pre><code>timesheets = self.get_billable_timesheets(obj)
  hours = 0
  for timesheet in timesheets:
      hours += timesheet.total_duration()
  </code></pre>
  <p>I get:
  <code>AttributeError: 'collections.OrderedDict' object has no attribute 'total_duration'</code></p>
  <p>What I don't understand is that I'm actually serializing the data already through the <code>get_billable_timesheets</code> method - why am I still receiving this error?</p>