Make LegacyGenericforeignKey work correctly on Django > 1.8.x

python/nav/models/fields.py

@@ -25,6 +25,7 @@
 from django import forms
 from django.db import models
 from django.db.models import signals
+from django.db.models.fields.mixins import FieldCacheMixin
 from django.core import exceptions
 from django.db.models import Q
 from django.apps import apps
@@ -146,7 +147,7 @@ def formfield(self, **kwargs):
 # this interfaces with Django model protocols, which generates unnecessary
 # pylint violations:
 # pylint: disable=W0201,W0212
-class LegacyGenericForeignKey(object):
+class LegacyGenericForeignKey(FieldCacheMixin):
     """Generic foreign key for legacy NAV database.
 
     Some legacy tables in NAV have generic foreign keys that look very much
@@ -155,16 +156,25 @@ class LegacyGenericForeignKey(object):
 
     """
 
+    # Field flags
+    auto_created = False
+    concrete = False
+    editable = False
+    hidden = False
+
+    is_relation = True
+    many_to_many = False
+    many_to_one = True
+    one_to_one = False
+    related_model = None
+    remote_field = None
+
     def __init__(self, model_name_field, model_fk_field, for_concrete_model=True):
         self.mn_field = model_name_field
         self.fk_field = model_fk_field
-        self.is_relation = True
-        self.many_to_many = False
         self.one_to_many = True
-        self.related_model = None
-        self.auto_created = False
-        self.for_concrete_model = for_concrete_model
         self.editable = False
+        self.for_concrete_model = for_concrete_model
 
     def __str__(self):
         modelname = getattr(self, 'mn_field')
@@ -175,14 +185,16 @@ def contribute_to_class(self, cls, name):
         """Add things to the model class using this descriptor"""
         self.name = name
         self.model = cls
-        self.cache_attr = "_%s_cache" % name
         cls._meta.private_fields.append(self)
 
         if not cls._meta.abstract:
             signals.pre_init.connect(self.instance_pre_init, sender=cls)
 
         setattr(cls, name, self)
 
+    def get_cache_name(self):
+        return self.name
+
     def instance_pre_init(self, signal, sender, args, kwargs, **_kwargs):
         """
         Handles initializing an object with the generic FK instead of
@@ -197,28 +209,22 @@ def instance_pre_init(self, signal, sender, args, kwargs, **_kwargs):
                 kwargs[self.mn_field] = None
                 kwargs[self.fk_field] = None
 
-    def is_cached(self, instance):
-        return hasattr(instance, self.cache_attr)
-
     def __get__(self, instance, instance_type=None):
         if instance is None:
             return self
 
-        try:
-            return getattr(instance, self.cache_attr)
-        except AttributeError:
-            rel_obj = None
+        rel_obj = self.get_cached_value(instance, default=None)
 
-            field = self.model._meta.get_field(self.mn_field)
-            table_name = getattr(instance, field.get_attname(), None)
-            rel_model = self.get_model_class(table_name)
-            if rel_model:
-                try:
-                    rel_obj = rel_model.objects.get(id=getattr(instance, self.fk_field))
-                except exceptions.ObjectDoesNotExist:
-                    pass
-            setattr(instance, self.cache_attr, rel_obj)
-            return rel_obj
+        field = self.model._meta.get_field(self.mn_field)
+        table_name = getattr(instance, field.get_attname(), None)
+        rel_model = self.get_model_class(table_name)
+        if rel_model:
+            try:
+                rel_obj = rel_model.objects.get(id=getattr(instance, self.fk_field))
+            except exceptions.ObjectDoesNotExist:
+                pass
+        self.set_cached_value(instance, rel_obj)
+        return rel_obj
 
     def __set__(self, instance, value):
         if instance is None:
@@ -232,7 +238,7 @@ def __set__(self, instance, value):
 
         setattr(instance, self.mn_field, table_name)
         setattr(instance, self.fk_field, fkey)
-        setattr(instance, self.cache_attr, value)
+        self.set_cached_value(instance, value)
 
     @staticmethod
     def get_model_name(obj) -> str: