old_contents: from rest_framework import serializers
  from .models import Activity, Datapoint
  
  
  class ActivitySerializer(serializers.HyperlinkedModelSerializer):
      class Meta:
          model = Activity
          fields = ('user', 'full_description', 'units', 'url')
  
  
  class DatapointSerializer(serializers.HyperlinkedModelSerializer):
      class Meta:
          model = Datapoint
          fields = ('user', 'activity', 'reps', 'timestamp', 'url')
  
new_contents: from rest_framework import serializers
  from .models import Activity, Datapoint
  
  
  class ActivitySerializer(serializers.HyperlinkedModelSerializer):
      class Meta:
          model = Activity
          fields = ('id', 'user', 'full_description', 'units', 'url')
  
  
  class DatapointSerializer(serializers.HyperlinkedModelSerializer):
      class Meta:
          model = Datapoint
          fields = ('id', 'user', 'activity', 'reps', 'timestamp', 'url')
  
subject: Add 'id' to both Serializers