models.py

from django.db import models


class Car(models.Model):
    name = models.CharField(max_length=255)

    def __str__(self):
        return self.name


class CarModel(models.Model):
    name = models.CharField(max_length=255)
    car = models.ForeignKey(Car, on_delete=models.SET_NULL, blank=True, null=True)
    
    def __str__(self):
        return self.name