How to make a model field equal to a queryset result

February 17, 2022

I am new to django and coding in general. I have this project that I am trying to code thats basically an auctions website.

I am facing some difficulties structuring the models though.

Here are 2 models of all models

class Listing(models.Model):
    title = models.CharField(max_length=64)
    image = models.URLField(null=True, blank=True)
    description = models.CharField(max_length=64)
    starting_price = models.DecimalField(max_digits=7, decimal_places=2)    

    current_price = #highest bid price
    bids_count = #count of bids

    lister = models.ForeignKey(User, on_delete=models.CASCADE, related_name="listings")

    def __str__(self):
        return f"Title: {self.title} ({self.id}), Lister: {self.lister.username}"


class Bid(models.Model):
    listing = models.ForeignKey(Listing, on_delete=models.CASCADE, related_name="bids")
    bidder = models.ForeignKey(User, on_delete=models.SET_NULL, null=True, related_name="bids")
    amount = models.DecimalField(max_digits=7, decimal_places=2)
    time = models.DateTimeField(auto_now_add=True)

    def __str__(self):
        return f"Bid by {self.bidder.username} on {self.listing.title} ({self.listing.id}) for an amount of {self.amount}"

I am trying to let the current price equal to the highest bid of that listing

current_price = Listing.bid.objects.all().aggregate(Max("amount"))

And count the number of bids

bids_count = Listing.bid.count()

I know we cant place querysets inside models fields like I have done but i did it to demonstrate my issue.

Surely there is some way around this but I just cant figure it out.

>Solution :

As you said, you cannot put those fields "as-is" in your model. The easiest/quickest way to solve it would be to use a property:

class Listing(models.Model):
    # ... rest of the class

    @property
    def bids_count(self):
        return self.bids.count()

    @property
    def current_price(self):
        return self.bids.all().aggregate(Max("amount"))

    # ... rest of the class

now, be aware that this will do fine when you work with a single instance. This will not be performant if you are looping over a list of Listing instances and display those properties, because it will trigger a new db query each time you access those properties (so these values are fetched in a lazy way)

The best workaround in my opinion would be to use a custom manager, as follow:

class ListingQuerySet(models.QuerySet):

    def with_bids_count(self):
        return self.annotate(bids_count=Count('bids'))

    def with_current_price(self):
        return self.annotate(current_price=Subquery(Bid.objects.filter(listing=OuterRef('pk')).annotate(max=Max('amount')).values('max')[:1]))

class Listing(models.Model):
    
    objects = ListingQuerySet.as_manager()

    # ... rest of the class

# Use it like this in your code

for listing in Listing.objects.with_bids_count().with_current_price():
    print(listing.current_price)

This previous method is more advanced for someone new to Django/coding (especially with the subquery). You’ll be able to read more about all this in the documentation:

Note that I didn’t try the code