[1.8.x] Fixed #24287 -- Added friendly error if a model is in a models.py outside an installed app.

django/db/models/base.py

@@ -2,34 +2,41 @@ from __future__ import unicode_literals
 
 import copy
 import inspect
-from itertools import chain
 import sys
 import warnings
+from itertools import chain
 
 from django.apps import apps
 from django.apps.config import MODELS_MODULE_NAME
 from django.conf import settings
 from django.core import checks
-from django.core.exceptions import (FieldDoesNotExist, ObjectDoesNotExist,
-    MultipleObjectsReturned, FieldError, ValidationError, NON_FIELD_ERRORS)
-from django.db import (router, connections, transaction, DatabaseError,
-    DEFAULT_DB_ALIAS, DJANGO_VERSION_PICKLE_KEY)
+from django.core.exceptions import (
+    NON_FIELD_ERRORS, FieldDoesNotExist, FieldError, ImproperlyConfigured,
+    MultipleObjectsReturned, ObjectDoesNotExist, ValidationError,
+)
+from django.db import (
+    DEFAULT_DB_ALIAS, DJANGO_VERSION_PICKLE_KEY, DatabaseError, connections,
+    router, transaction,
+)
 from django.db.models import signals
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.deletion import Collector
 from django.db.models.fields import AutoField
-from django.db.models.fields.related import (ForeignObjectRel, ManyToOneRel,
-    OneToOneField, add_lazy_relation)
+from django.db.models.fields.related import (
+    ForeignObjectRel, ManyToOneRel, OneToOneField, add_lazy_relation,
+)
 from django.db.models.manager import ensure_default_manager
 from django.db.models.options import Options
 from django.db.models.query import Q
-from django.db.models.query_utils import DeferredAttribute, deferred_class_factory
+from django.db.models.query_utils import (
+    DeferredAttribute, deferred_class_factory,
+)
 from django.utils import six
 from django.utils.deprecation import RemovedInDjango19Warning
 from django.utils.encoding import force_str, force_text
 from django.utils.functional import curry
 from django.utils.six.moves import zip
-from django.utils.text import get_text_list, capfirst
+from django.utils.text import capfirst, get_text_list
 from django.utils.translation import ugettext_lazy as _
 from django.utils.version import get_version
 
@@ -115,8 +122,14 @@ class ModelBase(type):
                     app_label_index = package_components.index(MODELS_MODULE_NAME) + 1
                 except ValueError:
                     app_label_index = 1
-                kwargs = {"app_label": package_components[app_label_index]}
-
+                try:
+                    kwargs = {"app_label": package_components[app_label_index]}
+                except IndexError:
+                    raise ImproperlyConfigured(
+                        'Unable to detect the app label for model "%s." '
+                        'Ensure that its module, "%s", is located inside an installed '
+                        'app.' % (new_class.__name__, model_module.__name__)
+                    )
             else:
                 kwargs = {"app_label": app_config.label}