'Remove the passed app from this collection, returning a boolean
indicating whether a successful deletion took place.'

def remove_app(self, app):

    try:
        membership = self.membership_class.objects.get(obj=self, app=app)
    except self.membership_class.DoesNotExist:
        return False
    else:
        membership.delete()
        index_webapps.delay([app.pk])
        return True
