class CarParamType(models.Model):
      name = models.CharField(max_length=50)
      unit = models.CharField(max_length=10)

class CarParam(models.Model):
      car = models.ForeignKey(Car)
      car_param_type = models.ForeignKey('CarParamType')
      value = models.CharField(max_length=20)

class DriveType(models.Model):
      name = models.CharField(max_length=20)

class Car(models.Model):
      vin = models.CharField(max_length=20)

class CarParamAdmin(admin.TabularInline):
      model = CarParam


@admin.register(Car)
  class CarAdmin(CustomModelAdmin):
      inlines = [
          CarParamAdmin,
      ]

if car_param_type.name == 'drive':
    form = forms.ModelChoiceField(queryset=DriveType.objects.all())