          obj = getattr(obj, self.source or field_name)
  
          # If the object has an "all" method, assume it's a relationship
          if is_simple_callable(getattr(obj, 'all', None)):
              return [self.to_native(item) for item in obj.all()]
  
          return self.to_native(obj)
  
 
</s>
        if is_simple_callable(obj):
            obj = obj()

          obj = getattr(obj, self.source or field_name)
  
        if is_simple_callable(obj):
            obj = obj()

          # If the object has an "all" method, assume it's a relationship
          if is_simple_callable(getattr(obj, 'all', None)):
              return [self.to_native(item) for item in obj.all()]
  
          return self.to_native(obj)
  
keep add keep keep keep keep keep keep
 Fix related serializers with source argument that resolves to a callable
rest_framework/serializers.py