Custom validation in Django admin form

Django applies the validation rules defined in the Model or Form. But sometimes we might need custom validation in Django admin forms.

Let’s assume that we are developing an app that’ll store all of the countries data. We’ll also feature 5 of them in the homepage of the app. Let’s consider the following Model

file: models.py

from django.db import models

class Country(models.Model):
    name = models.CharField(max_length=30)
    population = models.IntegerField()
    is_independent = models.BooleanField()
    featured = models.BooleanField()

Now if you register this Model, you’ll get auto generated form in Django Admin:
file: admin.py

from myapp.models import Country
from django.contrib import admin

admin.site.register(Country)

But it’ll never know that you wanted to add at most 5 countries as Featured country. After the 5th featured country if you add another country as featured country it won’t warn you, it’ll just let you do it. But with custom validation you can do what you really wanted.

Create a form (here CountryForm) that extends django.forms.ModelForm. then create a method clean_[field_name] in that form. Here, we wanted to validate the featured field, so the method name would be clean_featured. And the posted field value is self.cleaned_data['featured']. You can also raise an error message, see the code below:

file: admin.py

from django import forms
from myapp.models import Country
from django.contrib import admin

class CountryForm(forms.ModelForm):

    class Meta:
        model = Country

    def clean_featured(self):

        featuredCount = Country.objects.filter(featured=True).count()

        if featuredCount >= 5 and self.cleaned_data['featured'] is True:
            raise forms.ValidationError("5 Countries can be featured at most!")
        return self.cleaned_data['featured']


class CountryAdmin(admin.ModelAdmin):
    form = CountryForm

admin.site.register(Country, CountryAdmin)