Question

So let’s say I have three models: User, CarBrand, CarModel, CarCharacteristics. The CarCharacteristics model describes how a user rates the characteristics of a car. So there can be many CarCharacteristics objects for a user, and a user can have many CarCharacteristics.

This is what my models look like:

class User(models.Model):
    id = models.AutoField(primary_key=True)

class CarBrand(models.Model):
    idcar_brand = models.AutoField(primary_key=True)

class CarModel(models.Model):
    idcar_model = models.AutoField(primary_key=True)
    car_brand = models.ForeignKey(CarBrand, null=True, on_delete=models.SET_NULL)

class CarCharacteristics(models.Model):
    idcar_characteristics = models.AutoField(primary_key=True)
    user = models.ForeignKey(User, on_delete=models.PROTECT)
    car_model = models.ForeignKey(CarModel, on_delete=models.PROTECT)

What I would like to do is to get all CarBrand objects where a User has a CarCharacteristics. This is what am I doing right now:

car_chars = CarCharacteristics.objects.filter(user=user_id)
car_brands_ids = []
for car_char in car_chars:
    brand_id = car_char.car_model.car_brand.idcar_brand
    if brand_id not in car_brands_ids:
        car_brands_ids.append(brand_id)
brands = CarBrand.objects.filter(idcar_brand__in=brand_ids)

Is there a simpler way of doing this? And how do I get all the CarModels where there exists a CarCharacteristics?

Answer 1

I think I figured it out. I think this works:

user_id = request.user.pk
brands = CarBrand.objects.exclude(~Q(carmodel__carcharacteristics__user=user_id))

EDIT

Is what I have above also equivalent to this:

user_id = request.user.pk
car_models = CarModel.objects.filter(carcharacteristics__user=user_id)
brands = CarBrand.objects.filter(carmodel__in=car_models).distinct()