How can django store the pk/id of an object in modelchoicefield?

Question

Django

In model.py, something like this model

class OrderItem(models.Model):
    description = models.CharField(
        blank=True,
        null=True,
        max_length=255)
    car_id = models.PositiveIntegerField(blank=True,
        null=True,)

admin.py

class OrderItemForm(forms.ModelForm):
  # class Meta:
    # model = OrderItem
  # car_id = forms.ModelChoiceField(queryset=Car.objects.filter(pk=self.car_id))
  def __init__(self, *args, **kwargs):
    super(OrderItemForm, self).__init__(*args, **kwargs)
                self.fields['car_id'] = forms.ModelChoiceField(queryset=Car.objects.all())

self.fields['car_id'] = forms.ModelChoiceField(queryset=Car.objects.all())

class OrderItemAdmin(admin.TabularInline):
  # raw_id_fields = ("id",)
  form = OrderItemForm
  model = OrderItem
  exclude = ('hash', 'locked','options_hash',  'description2')
  # fields = ("description", "price", "quantity", 'car_id')

  def __init__(self, *args, **kwargs):
    super(OrderItemAdmin, self).__init__(*args, **kwargs)

Where and what event do I need to override? tried save, save_model, save_related, but validation on integer happens somewhere earlier. Please help me figure out where, thanks!