wagtail_SinglePageInstanceMixin.py

from wagtail.wagtailcore.models import get_page_models
from wagtail.wagtailcore.utils import resolve_model_string


class SinglePageInstanceMixin(object):

    @classmethod
    def clean_parent_page_types(cls):
        """
        Rewrite wagtailcore.models.Page.clean_parent_page_models to ensure that
        this is recalculated every time it's needed, rather than only the first
        time. It returns parent_page_models, normalised as model classes, if so
        defined on the model, otherwise all page-derived models, but only if no
        instance of this Page-derived class currently exists. Otherwise returns
        [], thus enforcing (in view logic) that only one instance of this class
        can exist at a time.
        """
        if cls.objects.all().exists():
            return []
        else:
            parent_page_types = getattr(cls, 'parent_page_types', None)
            if parent_page_types is None:
                # if parent_page_types is not specified on the Page class, allow all page types as subpages
                res = get_page_models()
            else:
                res = [
                    resolve_model_string(model_string, cls._meta.app_label)
                    for model_string in parent_page_types
                ]
                for model in res:
                    if not issubclass(model, Page):
                        raise LookupError("%s is not a Page subclass" % model)
            return res