PolymorphicRelatedQuerySet which allows select_related() to return polymorphic child models instead of base model

polymorphic_related_queryset.py

"""
PolymorphicRelatedQuerySet which allows select_related()  to return polymorphic child models instead of base model
Based on code by AndySun25 at: https://gist.github.com/AndySun25/157da4195c139a38ec3d609a277773cb
To address issue: https://github.com/django-polymorphic/django-polymorphic/issues/244
Added support for Polymorphic Proxy Models
"""

import functools
from django.db import models
from django.db.models.constants import LOOKUP_SEP
from django.db.models.query import ModelIterable
from django.apps import apps
from polymorphic.models import PolymorphicModel
from polymorphic.query import transmogrify


def rgetattr(obj, attr, separator='.'):
    """Recursive getattr to retrieve attributes of nested objects."""
    return functools.reduce(getattr, [obj] + attr.split(separator))


def rsetattr(obj, attr, value, separator='.'):
    """Recursively getattr to fetch final object layer before using setattr."""
    attrs = attr.split(separator)
    setattr(functools.reduce(getattr, attrs[:-1], obj), attrs[-1], value)


def is_polymorphic_subclass(super_cls, sub_cls):
    try:
        return (issubclass(sub_cls, super_cls) and
                sub_cls != models.Model and
                sub_cls != super_cls and
                sub_cls != PolymorphicModel and not
                sub_cls._meta.abstract)
    except AttributeError:
        return False


def polymorphic_iterator(*fields):
    """
    Creates a custom iterator which replaces the referenced instances with child versions
    :param fields:
    :return:
    """
    class PolymorphicModelIterable:
        def __init__(self, *args, **kwargs):
            self.iterable = ModelIterable(*args, **kwargs)

        def __iter__(self):
            for obj in self.iterable:
                for field in fields:
                    # Must get recursively in case our related polymorphic model is nested.
                    instance = rgetattr(obj, field, separator=LOOKUP_SEP)
                    real_instance_class = instance.polymorphic_ctype.model_class()
                    if real_instance_class._meta.proxy:
                        # Change class of proxy model
                        real_instance = transmogrify(real_instance_class, instance)
                    else:
                        real_instance_name = instance.polymorphic_ctype.model
                        # We must copy the field cache for the base_model instance to the real instance
                        # else additional data from select_related will be lost.
                        real_instance = instance._state.fields_cache.pop(real_instance_name)
                        real_instance._state.fields_cache = instance._state.fields_cache
                    # Same recursion goes here for setting the related object.
                    rsetattr(obj, field, real_instance, separator=LOOKUP_SEP)
                yield obj

    return PolymorphicModelIterable


class PolymorphicRelatedQuerySetMixin:
    """
    A class with a relationship to a polymorphic model should use this queryset
    TODO: Override normal select_related() so the functionality works automatically/seamlessly
    """

    def _get_nested_base_model(self, field):
        field_parts = field.split(LOOKUP_SEP)
        model = self.model
        for part in field_parts:
            field = getattr(model, part)
            # Should find a better solution to determine the related model than below.
            try:
                # In case of forward related descriptors.
                model = field.field.related_model
            except AttributeError:
                # In case of reverse related descriptors.
                model = field.related.related_model

        return model

    def select_polymorphic_related(self, *fields):
        """
        Specify fields that should be cast to the real polymorphic class.
        """
        subclass_names = []
        # Add references to all child models in select_related, e.g. reference__childA, reference__childB etc
        if fields:
            for field_name in fields:
                polymorphic_parent_model = self._get_nested_base_model(field_name)
                # This is somewhat a replication of PolymorphicModel._get_inheritance_relation_fields_and_models(),
                # but it's necessary to do this unless we want to instantiate a base_model instance for every
                # query made. Would be a consideration to perhaps cache the results.
                for model in apps.get_models():
                    if is_polymorphic_subclass(polymorphic_parent_model, model):
                        parent_link_field = model._meta.parents[polymorphic_parent_model]
                        if parent_link_field is None:  # Proxy model
                            pass
                        elif parent_link_field.remote_field.related_name:
                            subclass_names.append(parent_link_field.remote_field.related_name)
                        else:
                            subclass_names.append('{}__{}'.format(field_name, model.__name__.lower()))
                # We also need to add the polymorphic_ctype field name
                polymorphic_ctype_field_name = polymorphic_parent_model.polymorphic_internal_model_fields[0]
                subclass_names.append('{}__{}'.format(field_name, polymorphic_ctype_field_name))

        self._iterable_class = polymorphic_iterator(*fields)
        return self.select_related(*subclass_names)


class PolymorphicRelatedQuerySet(PolymorphicRelatedQuerySetMixin, models.QuerySet):
    pass

I can't seem to get this code working for a situation where I have a StandardModel, a PolymorphicBaseModel (related to standard model on related_name "myrelation"), and multiple PolymorphicChildren.

I only work with the Children models, so when I have my StandardModel, I would like to be able to do StandardModel.objects.get(id=1).select_polymorphic_related("myrelation") and have it grab all the various Children with their appropriate attributes and models instead of the default of returning PolymorphicBaseModels with incomplete attributes and methods.

I managed to get your code above partially implemented, but it kept running into various issues that I was unable to parse.

Is the desired scenario I described something this code can do, or does it need some tweaks?

Thanks for any help you can provide!

Is the problem perhaps because you are doing .get() before .select_polymorphic_related()? Try the other way around