old_contents: from rest_framework import serializers
  from activities.models import Activity, Episode
  from clubs.models import Club
  
  class EpisodeSerializer(serializers.ModelSerializer):
      class Meta:
          model = Episode
          fields = ('start_datetime', 'end_datetime')
  
  class ClubSerializer(serializers.ModelSerializer):
      class Meta:
          model = Club
          fields = ('pk', 'name', 'english_name', 'email', 'coordinator', 'gender', 'city')
  
  class ActivitySerializer(serializers.ModelSerializer):
      primary_club = ClubSerializer(read_only=True)
      secondary_clubs = ClubSerializer(many=True, read_only=True)
      episode_set = EpisodeSerializer(many=True, read_only=True)
      class Meta:
          model = Activity
          fields = ('pk', 'name', 'primary_club', 'secondary_clubs', 'public_description', 'episode_set', 'gender')
  
new_contents: from rest_framework import serializers
  from activities.models import Activity, Episode
  from clubs.models import Club
  
  class EpisodeSerializer(serializers.ModelSerializer):
      class Meta:
          model = Episode
          fields = ('pk', 'start_datetime', 'end_datetime')
  
  class ClubSerializer(serializers.ModelSerializer):
      class Meta:
          model = Club
          fields = ('pk', 'name', 'english_name', 'email', 'coordinator', 'gender', 'city')
  
  class ActivitySerializer(serializers.ModelSerializer):
      primary_club = ClubSerializer(read_only=True)
      secondary_clubs = ClubSerializer(many=True, read_only=True)
      episode_set = EpisodeSerializer(many=True, read_only=True)
      class Meta:
          model = Activity
          fields = ('pk', 'name', 'primary_club', 'secondary_clubs', 'public_description', 'episode_set', 'gender')
  
subject: Add pk for episodes.