<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_internal_value_single(self, data, serializer):
 """Return the underlying object, given the serialized form."""

        related_model = serializer.Meta.model
        if isinstance(data, related_model):
            return data
        try:
            instance = related_model.objects.get(pk=data)
        except related_model.DoesNotExist:
            raise ValidationError(
                "Invalid value for '%s': %s object with ID=%s not found" %
                (self.field_name, related_model.__name__, data)
            )
        return instance