Wagtail page type validator

models.py

from django.core.exceptions import ValidationError
from django.utils.deconstruct import deconstructible

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


@deconstructible
class PageTypeValidator(object):
    """
    Usage:

    class HomePage(Page):
        related_page = models.ForeignKey(
            'wagtailcore.Page', null=True, blank=True, on_delete=models.SET_NULL, related_name='+',
            validators=[PageTypeValidator('blog.BlogIndex', 'blog.BlogPost')]
        )

    """
    def __init__(self, *models):
        self.models = models

    def __call__(self, page_id):
        page = Page.objects.get(id=page_id).specific
        if not any(isinstance(page, resolve_model_string(model)) for model in self.models):
            raise ValidationError("Please select a valid page type")

    def __eq__(self, other):
        return self.models == other.models