Limit number of Django model instances

admin.py

from django.contrib import admin
from example.models import Example


class ExampleAdmin(admin.ModelAdmin):
    """
    Don't allow addition of more than one model instance in Django admin
    See: http://stackoverflow.com/a/12469482
    """
    def has_add_permission(self, request):
        if self.model.objects.count() > 0:
            return False
        else:
            return True

admin.site.register(Example, ExampleAdmin)

models.py

from django.db import models
from django.core.exceptions import ValidationError


class Example(models.Model):
    def clean(self):
        """
        Throw ValidationError if you try to save more than one model instance
        See: http://stackoverflow.com/a/6436008
        """
        model = self.__class__
        if (model.objects.count() > 0 and
                self.id != model.objects.get().id):
            raise ValidationError(
                "Can only create 1 instance of %s." % model.__name__)