Fixed #21718 -- Renamed has_app to is_installed.

        app_models[model_name] = model

        self.clear_cache()

    def has_app(self, app_name):

    def is_installed(self, app_name):

        """

        Checks whether an application with this name exists in the registry.

        The default implementation checks that admin and contenttypes apps are

        installed, as well as the auth context processor.

        """

        if not apps.has_app('django.contrib.admin'):

        if not apps.is_installed('django.contrib.admin'):

            raise ImproperlyConfigured("Put 'django.contrib.admin' in your "

                "INSTALLED_APPS setting in order to use the admin application.")

        if not apps.has_app('django.contrib.contenttypes'):

        if not apps.is_installed('django.contrib.contenttypes'):

            raise ImproperlyConfigured("Put 'django.contrib.contenttypes' in "

                "your INSTALLED_APPS setting in order to use the admin application.")

        if 'django.contrib.auth.context_processors.auth' not in settings.TEMPLATE_CONTEXT_PROCESSORS:

register = Library()

if apps.has_app('django.contrib.staticfiles'):

if apps.is_installed('django.contrib.staticfiles'):

    from django.contrib.staticfiles.templatetags.staticfiles import static

else:

    from django.templatetags.static import static

def skipUnlessAuthIsInstalled(func):

    return skipUnless(

        apps.has_app('django.contrib.auth'),

        apps.is_installed('django.contrib.auth'),

        "django.contrib.auth isn't installed")(func)

    response_redirect_class = http.HttpResponsePermanentRedirect

    def __init__(self):

        if not apps.has_app('django.contrib.sites'):

        if not apps.is_installed('django.contrib.sites'):

            raise ImproperlyConfigured(

                "You cannot use RedirectFallbackMiddleware when "

                "django.contrib.sites is not installed."