old_contents: from core.models import ProviderMachine
  from rest_framework import serializers
  from ..summaries import ImageSummarySerializer, ProviderSummarySerializer, UserSummarySerializer
  
  
  class ProviderMachineSerializer(serializers.HyperlinkedModelSerializer):
      image = ImageSummarySerializer(source='application')
      provider = ProviderSummarySerializer()
      created_by = UserSummarySerializer()
  
      class Meta:
          model = ProviderMachine
          view_name = 'api_v2:providermachine-detail'
          fields = ('id', 'url', 'image', 'provider', 'created_by', 'start_date', 'end_date')
  
new_contents: from core.models import ProviderMachine
  from rest_framework import serializers
  from ..summaries import ImageSummarySerializer, ProviderSummarySerializer, UserSummarySerializer
  
  
  class ProviderMachineSerializer(serializers.HyperlinkedModelSerializer):
      image = ImageSummarySerializer(source='application')
      provider = ProviderSummarySerializer(source='instance_source.provider')
      created_by = UserSummarySerializer(source='instance_source.created_by')
      start_date = serializers.DateTimeField(source='instance_source.start_date')
      end_date = serializers.DateTimeField(source='instance_source.end_date')
  
      class Meta:
          model = ProviderMachine
          view_name = 'api_v2:providermachine-detail'
          fields = ('id', 'url', 'image', 'provider', 'created_by', 'start_date', 'end_date')
  
subject: fix provider_machine serializer to use sources