import ContentType via property / django_apps.get_model (#630)

* import ContentType via property / django_apps.get_model instead of direct import

* CHANGES.rst

Author: erikvw

CHANGES.rst

@@ -5,6 +5,8 @@ Unreleased
 ----------
 - Add simple filtering if provided a minutes argument in `clean_duplicate_history` (gh-606)
 - Add setting to convert `FileField` to `CharField` instead of `TextField` (gh-623)
+- import model `ContentType` in `SimpleHistoryAdmin` using `django_apps.get_model`
+  to avoid possible `AppRegistryNotReady` exception (gh-630)
 
 2.8.0 (2019-12-02)
 ------------------

simple_history/admin.py

@@ -2,12 +2,12 @@
 
 import django
 from django import http
+from django.apps import apps as django_apps
 from django.conf import settings
 from django.conf.urls import url
 from django.contrib import admin
 from django.contrib.admin import helpers
 from django.contrib.admin.utils import unquote
-from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import PermissionDenied
 from django.shortcuts import get_object_or_404, render
 from django.urls import reverse
@@ -80,7 +80,9 @@ def history_view(self, request, object_id, extra_context=None):
                 for list_entry in action_list:
                     setattr(list_entry, history_list_entry, value_for_entry(list_entry))
 
-        content_type = ContentType.objects.get_by_natural_key(*USER_NATURAL_KEY)
+        content_type = self.content_type_model_cls.objects.get_by_natural_key(
+            *USER_NATURAL_KEY
+        )
         admin_user_view = "admin:%s_%s_change" % (
             content_type.app_label,
             content_type.model,
@@ -196,7 +198,9 @@ def history_form_view(self, request, object_id, version_id, extra_context=None):
             "has_absolute_url": False,
             "form_url": "",
             "opts": model._meta,
-            "content_type_id": ContentType.objects.get_for_model(self.model).id,
+            "content_type_id": self.content_type_model_cls.objects.get_for_model(
+                self.model
+            ).id,
             "save_as": self.save_as,
             "save_on_top": self.save_on_top,
             "root_path": getattr(self.admin_site, "root_path", None),
@@ -216,3 +220,9 @@ def save_model(self, request, obj, form, change):
         """Set special model attribute to user for reference after save"""
         obj._history_user = request.user
         super(SimpleHistoryAdmin, self).save_model(request, obj, form, change)
+
+    @property
+    def content_type_model_cls(self):
+        """Returns the ContentType model class.
+        """
+        return django_apps.get_model("contenttypes.contenttype")