How to compare two integer fields in a model method in django?

The problem is not in your method but rather your validator, specifically MaxValueValidator(total_slots) won't work because MaxValueValidator needs to be passed an integer and not a model field. If you do want to set up some validation for that you can create a CheckConstraint for it:

from django.db import models class YourModel(models.Model): total_slots = models.IntegerField(default=0, help_text="Total number of vehicle slots available for allocation.") allocated_slots = models.IntegerField(default=0, validators=[MinValueValidator(0)], ) class Meta: constraints = [ models.CheckConstraint( check=models.Q(allocated_slots__lte=models.F("total_slots")), name="allocated_slots_lte_total_slots", ) ] 

Your error is conducted in the MaxValueValidator, this validator only receives integers but in that case you pass a model field. You can use several methods to implement this validation.

Add this method in your model:

def save(self, *args, **kwargs): if self.allocated_slots > self.total_slots: raise ValidationError("Your error message") return super().save(*args, **kwargs) 

You can use this clean method in that case you have to call the full_clean() method before save.

def clean(self, *args, **kwargs): if self.allocated_slots > self.total_slots: raise ValidationError("Your error message")