Filter condition on nested attribute

[ck-on-github]

Hello,

In my pynamodb model, I have an attribute, which can be an instance of one of the sub-classes of a polymorphic map attribute like:

class ParentClass(MapAttribute):
    cls = DiscriminatorAttribute()

class ChildClass1(ParentClass, discriminator = cls.__name__):
    filter_attr = UnicodeAttribute()

class ChildClass2(ParentClass, discriminator = cls.__name__):
    other_attr = UnicodeAttribute()

class MyModel(Model):
   ...
   attr = ParentClass() 

If I define a filter condition on MyModel.attr.filter_attr like MyModel.attr.filter_attr == value, I get the run time error

    def __getattr__(self, attr: str) -> _VT:
        # This should only be called for "raw" (i.e. non-subclassed) MapAttribute instances.
        # MapAttribute subclasses should access attributes via the Attribute descriptors.
        if self.is_raw() and self._is_attribute_container():
            try:
                return self.attribute_values[attr]
            except KeyError:
                pass
>       raise AttributeError("'{}' has no attribute '{}'".format(self.__class__.__name__, attr))
E       AttributeError: 'ParentClass' has no attribute 'filter_attr'

which makes sense because attr is of type ParentClass, which only has cls attribute.

My question is, is there a way to define a filter on a nested attribute in a pynamodb model, where the nested attribute is an instance of a child class of a polymorphic MapAttribute parent class?

[ikonst]

I can't think of a good way to do this right now. You can probably go lower-level by doing something like MyModel.attr['filter_attr'] == {'S': 'foobar'}, which is not ideal syntax-wise (you lose the knowledge of filter_attr's type and end up performing serialization yourself),.

[ck-on-github]

Thanks @ikonst for your prompt answer.

If I do MyModel.attr['filter_attr'] == value when ParentClass inherits MapAttribute, I get AttributeError: 'ParentClass' has no attribute 'filter_attr' again.

if ParentClass inherits DynamicMapAttribute, this seems to work though.

is there anything else I can try to make this work with MapAttribute?

[ikonst]

Oh, sorry, I guess you have to do something like Path(MyModel.attr.attr_path + ['filter_attr']). Again, I wish there was a nicer way.