new_contents: from models import ContentMetadata, File, Format
  from rest_framework import serializers
  
  
  class ContentMetadataSerializer(serializers.ModelSerializer):
      class Meta:
          model = ContentMetadata
          fields = ('content_id', 'title', 'description', 'kind', 'slug', 'total_file_size', 'available', 'license', 'prerequisite', 'is_related', 'parent')
  
  
  class FormatSerializer(serializers.ModelSerializer):
      class Meta:
          model = Format
          fields = ('available', 'format_size', 'quality', 'contentmetadata', 'mimetype')
  
  
  class FileSerializer(serializers.ModelSerializer):
      class Meta:
          model = File
          fields = ('checksum', 'extension', 'available', 'file_size', 'content_copy', 'format')
  
subject: add serializers for the content app models