Limit a single record in model for django app?

Try this:

class MyModel(models.Model):
    onefield = models.CharField('The field', max_length=100)

class MyModelAdmin(admin.ModelAdmin):
  def has_add_permission(self, request):
    # if there's already an entry, do not allow adding
    count = MyModel.objects.all().count()
    if count == 0:
      return True

    return False

An easy way is to use the setting's name as the primary key in the settings table. There can't be more than one record with the same primary key, so that will allow both Django and the database to guarantee integrity.

Overwriting has_add_permission works, but in the given examples it breaks the permissions system in Django(staff without necessary permissions can add settings). Here's a one that doesn't break it:

class SettingAdmin(admin.ModelAdmin):
    def has_add_permission(self, request):
        base_add_permission = super(SettingAdmin, self).has_add_permission(request)
        if base_add_permission:
            # if there's already an entry, do not allow adding
            count = Setting.objects.all().count()
            if count == 0:
                return True
        return False