Django: Validate ModelForm's Field dependent on another Field of that Model

Question

I have these two Django Models:

class Listing(models.Model):
    ...
    price = models.DecimalField(
        decimal_places=2, 
        max_digits=20, 
        default=0, 
        validators=[MinValueValidator(Decimal('0.01'))]
    )

class Bid(models.Model):
    ...
    listing = models.ForeignKey(Listing, default=None, on_delete=models.CASCADE)
    amount = models.DecimalField(
        decimal_places=2,
        max_digits=20, 
        default=0
        validators=[MinValueValidator(Decimal(listing.price))]
    )

I want to validate that Bid.amount > Bid.listing.price.

When I try to do it the with the help of MinValueValidator from django.core.validators I get this error:

    validators=[MinValueValidator(Decimal(listing.price))])
                                          ^^^^^^^^^^^^^
    AttributeError: 'ForeignKey' object has no attribute 'price'

I also tried referencing listing.price with a self attribute and got the following error:

validators=[MinValueValidator(Decimal(self.listing.price))])
                                          ^^^^
NameError: name 'self' is not defined

I know I can check the above condition in "views.py" once the Model.Form is submitted, but I wonder if there is a way to validate it with Django Validators.

Any help would be appreciated!

Answer 1

I figured it out by creating a custom validator in forms.py:

from .models import Listing, Bid
from django.forms import ModelForm
from decimal import Decimal
from django.core.exceptions import ValidationError


class BidForm(ModelForm):
    class Meta():
        model = Bid
        fields = ["amount"]

    def clean_amount(self):
        amount = self.cleaned_data["amount"]
        if Decimal(amount) < Decimal("0.01"):
            raise ValidationError(
                "Ensure this value is greater than or equal to 0.01."
            )
        elif Decimal(amount) <= Decimal(self.instance.listing.price):
            raise ValidationError("Ensure this value exceeds the price.")
        return amount

Then, in views.py:

from django.http import HttpResponseRedirect
from .models import Listing
from .forms import BidForm
from decimal import Decimal

def listing(request, listing_id):
    listing = Listing.objects.get(pk=listing_id)
    if request.method == "POST":
        form = BidForm(request.POST)
        form.instance.holder = request.user
        form.instance.listing = listing
        if form.is_valid():
            form.save()
            Listing.objects.filter(pk=listing_id).update(
                price=Decimal(request.POST["amount"])
            )
            return HttpResponseRedirect(
                f"/listing/{listing_id}", 
                {"form":form, "listing":listing}
            )
    else:
        form = BidForm()
    return render(request, "auctions/listing.html", {"form": form, "listing": listing})

Instead of validating Django Model's fields, I validate ModelForm submission with the help of clean_amount() method. Per Django's Documentation:

This method does any cleaning that is specific to that particular attribute, unrelated to the type of field that it is.

I hope this answer helps someone.