I upgraded a project from Django 1.8 to Django 2.1.3 (client requirements prevent a full update). Everything remains the same in models.py and the Django ORM query in views.py (aside from Python 3 updates and adding on_delete=models.CASCADE on some ForeignKey fields). The query works fine in Django 1.8, but in Django 2.1.3 it now raises the following error:

raise FieldError('Related Field got invalid lookup: {}'.format(lookups[0]))
django.core.exceptions.FieldError: Related Field got invalid lookup: carcategory

The main query is as follows:

cars = CarModel.objects.only(
'car__car_id',
'car__cardate',
'car__title',
'car__location',
'car__quote',
'car__key',
'car__cartype_id',
'maker__lastname',
'maker__firstname',
'maker__middlename',
'maker__nickname',
).select_related(
'car',
'maker',
).extra(
select={
'is_position_paper': 'cartype_id = 7',
'is_null_date': 'cardate IS NULL',
'shorttitle': extra,
},
).filter(**kwargs).distinct(sort_order_for_distinct, 'car__car_id').order_by(sort_order, 'car__car_id')

Before the upgrade, I could add the filter below without issues:

kwargs['car__carcategory__category__category_id'] = categoryId

Below are the relevant model definitions:

CarModel table:

class CarModel(models.Model):
    car_candidate_id = models.IntegerField(primary_key=True)
    car = models.ForeignKey(Car, on_delete=models.PROTECT)
    candidate = models.ForeignKey(Manufacturer, on_delete=models.PROTECT)
    created = models.DateTimeField(blank=True, null=True)
    modified = models.DateTimeField(blank=True, null=True)

    class Meta:
        managed = False
        db_table = 'car_model'

Car table:

class Car(models.Model):
    car_id = models.IntegerField(primary_key=True)
    cartype = models.ForeignKey('Cartype', on_delete=models.PROTECT)
    title = models.CharField(max_length=200)
    class Meta:
        managed = False
        db_table = 'car'

Carcategory table:

class Carcategory(models.Model):
    car_category_id = models.IntegerField(primary_key=True)
    car = models.ForeignKey(Car, on_delete=models.CASCADE)
    category = models.ForeignKey(Category, on_delete=models.CASCADE)
    class Meta:
        db_table = u'car_category'

Category table:

class Category(models.Model):
    category_id = models.SmallIntegerField(primary_key=True)
    name = models.CharField(max_length=255)
    description = models.CharField(max_length=2048, blank=True)
    release_id = models.SmallIntegerField()
    key = models.NullBooleanField(null=True)
    rank = models.SmallIntegerField(null=True)
    class Meta:
        db_table = u'category'
    def __unicode__(self):
        return unicode(self.category_id)

Any insights on why the filter on 'car__carcategory__category__category_id' now triggers an invalid lookup error in Django 2.1.3 would be appreciated.