add custom model manager

src/wagtail_personalisation/models.py

@@ -1,10 +1,12 @@
 from __future__ import absolute_import, unicode_literals
+
 import random
 
 from django import forms
 from django.conf import settings
 from django.core.validators import MaxValueValidator, MinValueValidator
 from django.db import models, transaction
+from django.db.models import F
 from django.template.defaultfilters import slugify
 from django.utils.encoding import python_2_unicode_compatible
 from django.utils.functional import cached_property
@@ -199,6 +201,16 @@ class Segment(ClusterableModel):
         return False
 
 
+class PersonalisablePageManager(models.Manager):
+
+    def canonicals(self):
+        return (
+            self.filter(
+                personalisable_canonical_metadata__canonical_page_id=F(
+                    'personalisable_canonical_metadata__variant__id'))
+        )
+
+
 class PersonalisablePageMetadata(ClusterableModel):
     """The personalisable page model. Allows creation of variants with linked
     segments.
@@ -219,6 +231,8 @@ class PersonalisablePageMetadata(ClusterableModel):
         on_delete=models.SET_NULL,
         null=True, blank=True)
 
+    objects = PersonalisablePageManager()
+
     @cached_property
     def has_variants(self):
         """Return a boolean indicating whether or not the personalisable page