code: def apply_customization(self, serializer, customization):
          
          # apply fields or exclude
          if customization.fields is not None:
              if len(customization.fields) == 0:
                  # customization fields are empty, set Meta.fields to '__all__'
                  serializer.Meta.fields = ALL_FIELDS
              else:
                  serializer.Meta.fields = customization.fields
          if customization.exclude is not None:
              serializer.Meta.exclude = customization.exclude
  
          # apply extra_kwargs
          if customization.extra_kwargs is not None:
              serializer.Meta.extra_kwargs = customization.extra_kwargs
  
          # apply validate_methods
          for method_name, method in customization.validate_methods.items():
              setattr(serializer, method_name, method)
docs: Applies fields customization to a nested or embedded DocumentSerializer.