Limit number of model instances to be created - django

Question

I have model from which I only want to create one instance, and no more instances should be allowed.

Is this possible? I\'ve got a feeling that I\'ve seen this done

Answer 1

@ncoghlan your solution is working fine, but not very user-friendly: the user has access to the creation form and will think he/she can use it, even though he/she will never be able to save it.

It's actually possible to combine it with Brendan's solution, which will hide the 'Add' button. Using Mixins for easy reuse:

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

class SingleInstanceMixin(object):
    """Makes sure that no more than one instance of a given model is created."""

    def clean(self):
        model = self.__class__
        if (model.objects.count() > 0 and self.id != model.objects.get().id):
            raise ValidationError("Can only create 1 %s instance" % model.__name__)
        super(SingleInstanceMixin, self).clean()

class Example(SingleInstanceMixin, models.Model):
    pass


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

class SingleInstanceAdminMixin(object):
    """Hides the "Add" button when there is already an instance."""
    def has_add_permission(self, request):
        num_objects = self.model.objects.count()
        if num_objects >= 1:
            return False
        return super(SingleInstanceAdminMixin, self).has_add_permission(request)

class ExampleAdmin(SingleInstanceAdminMixin, admin.ModelAdmin):
     model = Example