models.py

class Profile(models.Model):
    start_datetime = models.DateTimeField()
    some_timezone = models.CharField()

serializers.py

import pytz

class CustomDateTimeField(serializers.DateTimeField):
  def get_attribute(self, instance):
    # Save instance here for later usage
    self.instance = instance
    return super().get_attribute(instance)
  
  def to_representation(self, value):
      # how to access single `instance` here?
      # the only "instance" I can access here is `self.root.instance` which is queryset in case of many=True
      # If this is a single isntance serializer `self.root.isntance` is fine
      # If this is a many=True serializer nothing makes sense
      
      # This doesn't work
      instance = self.root.instance
      
      # This does work
      instance = self.instance
      
      value = pytz.timezone(instance.some_timezone).normalize(value)
      return super().to_representation(value)

class ProfileSerializer(serializers.ModelSerializer):
    start_datetime = CustomDateTimeField()
    
    class Meta:
        model = models.Profile
        fields = ('start_date',)

usecase.py

# This works
ProfileSerializer(Profile.objects.first()).data

# This doesn't work
# Error is: AttributeError: 'Manager' object has no attribute 'some_timezone' 
ProfileSerializer(Profile.objects.all(), many=True).data