admin.py

from django.contrib import admin
from .models import *


class AuctionAdmin(admin.ModelAdmin):
    list_display = ['title', 'current_bid_display', 'bid_count', 'expiry_date', 'active']

    def current_bid_display(self, obj):
        return "£{0}".format(obj.current_bid)


class BidAdmin(admin.ModelAdmin):
    list_display = ['auction', 'value', 'owner', 'created_at']
    list_filter = (
        ('auction',)
    )


admin.site.register(Auction, AuctionAdmin)
admin.site.register(Bid, BidAdmin)

models.py

from django.db import models
from django.contrib.auth.models import User


class AuctionManager(models.Manager):
    def active(self):
        return super().get_queryset().filter(active=True)

    def expired(self):
        return super().get_queryset().filter(active=False)


class Auction(models.Model):
    title = models.CharField(max_length=255)
    current_bid = models.FloatField()
    bid_count = models.IntegerField()
    created_at = models.DateTimeField(auto_now_add=True)
    live_date = models.DateTimeField()
    expiry_date = models.DateTimeField()
    active = models.BooleanField()
    owner = models.ForeignKey(User, on_delete=None)

    def __str__(self):
        return self.title

    def __unicode__(self):
        return self.title

    objects = models.Manager()
    AuctionManager = AuctionManager()


class BidManager(models.Manager):
    def current(self, auction):
        return super().get_queryset().filter(auction=auction).order_by('-created_at')


class Bid(models.Model):
    auction = models.ForeignKey(Auction, on_delete=None)
    value = models.FloatField()
    owner = models.ForeignKey(User, on_delete=None)
    created_at = models.DateTimeField(auto_now_add=True)

    objects = models.Manager()
    BidManager = BidManager()

signals.py

from django.dispatch import receiver
from django.db.models.signals import post_save
from .models import *


@receiver(post_save, sender=Bid)
def update_auction_totals_for_bid(sender, instance, created, **kwargs):
    if created:
        auction = instance.auction
        auction.bid_count = Bid.BidManager.current(auction).count()
        auction.current_bid = instance.value
        auction.save()