Django: Check for related objects and whether it contains data

Question

For example, I have two models in a car management system(webpage):

    class Brand(models.Model):
        brand_name= models.CharField(max_length=100, null=False)

    class Cars(models.Model):
        car_model= models.CharField(max_length=100, null=False)
        car_production_year= models.CharField(max_length=100, null=False)
        car_brand= models.ForeignKey(Brand, null=True, blank=True, default = None)

Now, I want to delete a Brand data from the car system. How can I check if this Brand has been used in another model or does that foreign key contains any data(as I have allowed null True for car_brand in Cars model).

PS: using this function:

self.model._meta.get_all_related_objects():

I got any related object in used in Brand model class. But, I don't know how to get if that related object contains any data.

Answer 1

From this question, for my code, this was faster:

for b in Brand.objects.filter(cars__isnull=True):
    # delete

If You have the ids (my case), I used this (60% faster than cars_set):

for brand_id in brand_id_set:
    if Brand.objects.filter(id=brand_id).filter(cars__isnull=True):
        # delete

I am using Django 1.11

Answer 2

Use exists(). It's designed to be used for exactly this type of situations:

for brand in Brand.objects.all():
    if not brand.cars_set.all().exists():
        # delete

Also it's almost always faster than any other type of check because of the way it is designed to work at the database level. You can read details of exists() behaviour in the docs