Pass on instance when using polymorphic serializers

[josebama]

I'm trying to understand the code in PolymorphicModelSerializer and I wonder if this is a bug or me just not understanding something, but in PolymorphicModelSerializer.to_internal_value, in the end, when in inits serializer_class like so: return serializer_class(data, context=self.context, partial=self.partial).to_internal_value(data), why is it passing data as the first argument?

For what I see, the init in BaseSerializer is expecting the first argument to be instance and the second one to be data: def __init__(self, instance=None, data=empty, **kwargs):. Wouldn't it make more sense for PolymorphicModelSerializer.to_internal_value to init serializer_class this other way: return serializer_class(self.instance, data, context=self.context, partial=self.partial).to_internal_value(data)?

That way the child serializer will keep the instance if there is any and will set the initial_data too.

[sliverc]

I have looked at the code as well and this seems invalid indeed. Usually this is not noticed as it only affects serializers which internally access self.instance in case of patch call. However this is a bug and wouldn't need to be changed as you have suggested.

PR is welcome.