An example on how to use F expressions

models.py

import uuid

from django.db import models

# assuming we have our User model implemented in the users app
from users.models import User

class Contest(models.Model):
    # I prefer using UUID field instead of auto increment for storing id
    id = models.UUIDField(primary_key=True, editable=False, default=uuid.uuid4)
    contest_name = models.CharField(max_length=100)
    start_time = models.DateTimeField(auto_now_add=True)
    end_time = models.DateTimeField(auto_now_add=True)
    
    def __str__(self):
        return self.contest_name


class ContestParticipant(models.Model):
    id = models.UUIDField(primary_key=True, editable=False, default=uuid.uuid4)
    contest = models.ForeignKey(Contest, on_delete=models.CASCADE)
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    registration_time = models.DateTimeField(auto_now_add=True)
    
    def __str__(self):
        return self.user.get_full_name()


# Querying ContestParticipant where registration_time > contest__end_time
from django.db.models import F

late_registrations = ContestParticipant.objects.filter(
    registration_time__gt=F('contest__end_time')