title: Django Rest Framework doesn't serialize SerializerMethodField
body: <p>I have 2 models:</p>
  
  <pre><code>from django.db import models
  
  STATUSES = (
      ('f', 'Finished'),
  )
  
  
  class Battery(models.Model):
      energy = models.CharField(max_length=10)
      current = models.CharField(max_length=10)
  
  
  class Charger(models.Model):
      status = models.CharField(max_length=1, choices=STATUSES)
  </code></pre>
  
  <p>And I want to create serializer that will serialize this 2 models together. My serializers.py:</p>
  
  <pre><code>from rest_framework import serializers
  from .models import Battery, Charger
  
  
  class BatterySerializer(serializers.ModelSerializer):
      class Meta:
          model = Battery
  
  
  class ChargerSerializer(serializers.ModelSerializer):
      status = serializers.SerializerMethodField()
  
      class Meta:
          model = Charger
  
      def get_status(self, obj):
          return obj.get_status_display()
  
  
  class DeviceSerializer(serializers.Serializer):
      battery = BatterySerializer()
      charger = ChargerSerializer()
      some_field = serializers.CharField()
  </code></pre>
  
  <p>Because Charger model has choices in status field I add SerializerMethodField for displaying full status. Then I create a view like this:</p>
  
  <pre><code>class DeviceView(APIView):
      def get(self, request, format=None):
          battery = Battery.objects.get(id=1)
          charger = Charger.objects.get(id=1)
          battery_serializer = BatterySerializer(battery)
          charger_serializer = ChargerSerializer(charger)
          serializer = DeviceSerializer(data={
              'battery': battery_serializer.data,
              'charger': charger_serializer.data,
              'some_field': 'some_text'
          })
          if serializer.is_valid():
              return Response(serializer.validated_data)
          else:
              return Response(status = 500)
  </code></pre>
  
  <p>But when I call this view it returns json with empty charger field:</p>
  
  <pre><code>{
      "battery": {
          "energy": "12",
          "current": "34"
      },
      "charger": {},
      "some_field": "some_text"
  }
  </code></pre>
  
  <p>But when I create a view that serialize only Charger model:</p>
  
  <pre><code>class ChargerView(APIView):
      def get(self, request, format=None):
          charger = Charger.objects.get(id=1)
          charger_serializer = ChargerSerializer(charger)
          return Response(charger_serializer.data)
  </code></pre>
  
  <p>It works and it returns this json:</p>
  
  <pre><code>{
      "id": 1,
      "status": "Finished"
  }
  </code></pre>
  
  <p>Why this happens? Where did I make a mistake?</p>