Check if related field is loaded when evaluating select_related queryset

django_related_utils.py

def is_related_loaded(instance: models.Model, related_model: Type[models.Model]) -> bool:
    """
    Checks if the model ``instance`` has a cached related field to a ``related_model``.
    The relation is cached as a consequence of calling the ``qs.select_related('related_model')``.

    This function is useful if you are doing ``qs.select_related('foo', 'bar')`` for backref relations,
    and there's only either of them available on the resulting object.

    >>> class Test(models.Model): ...
    >>> class Foo(models.Model): ...
    >>> class Bar(models.Model): ...
    >>> result = Test.objects.select_related('foo', 'bar').all()
    >>> [hasattr(obj, 'foo') for obj in result]  # will perform a query for each hasattr() where 'foo' is missing
    >>> [obj.foo for obj in result]  # might raise an exception, performing a query at the same time
    >>> [is_related_loaded(obj, Foo) for obj in result]  # inspects cache, no additional queries required
    """

    def rel_for_class(rel: models.OneToOneRel) -> bool:
        """We are looking for a relation to given ``related_model``."""
        return isinstance(rel, models.OneToOneRel) and rel.field.model is related_model

    def is_cached_rel(rel: models.OneToOneRel) -> bool:
        """
        The cached attribute can exist and be ``None`` if you try to select multiple related objects.
        We need to check the actual value.
        """
        return bool(getattr(instance, rel.get_cache_name(), False))

    fields = instance._meta.get_fields()
    return any(is_cached_rel(rel) for rel in fields if rel_for_class(rel))