Fixed #22236 -- Removed inappropriate usage of signals

Thanks Aymeric Augustin for the report and Tim Graham for the review.

django/db/models/base.py

@@ -7,20 +7,20 @@ import warnings
 
 from django.apps import apps
 from django.apps.config import MODELS_MODULE_NAME
-import django.db.models.manager  # NOQA: Imported to register signal handler.
 from django.conf import settings
 from django.core import checks
 from django.core.exceptions import (ObjectDoesNotExist,
     MultipleObjectsReturned, FieldError, ValidationError, NON_FIELD_ERRORS)
+from django.db import (router, transaction, DatabaseError,
+    DEFAULT_DB_ALIAS)
+from django.db.models.deletion import Collector
 from django.db.models.fields import AutoField, FieldDoesNotExist
 from django.db.models.fields.related import (ForeignObjectRel, ManyToOneRel,
     OneToOneField, add_lazy_relation)
-from django.db import (router, transaction, DatabaseError,
+from django.db.models.manager import ensure_default_manager
-    DEFAULT_DB_ALIAS)
+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.deletion import Collector
-from django.db.models.options import Options
 from django.db.models import signals
 from django.utils import six
 from django.utils.deprecation import RemovedInDjango19Warning
@@ -353,6 +353,7 @@ class ModelBase(type):
             cls.get_absolute_url = update_wrapper(curry(get_absolute_url, opts, cls.get_absolute_url),
                                                   cls.get_absolute_url)
 
+        ensure_default_manager(cls)
         signals.class_prepared.send(sender=cls)
 
 

django/db/models/manager.py

@@ -3,20 +3,18 @@ import inspect
 
 from django.db import router
 from django.db.models.query import QuerySet
-from django.db.models import signals
 from django.db.models.fields import FieldDoesNotExist
 from django.utils import six
 from django.utils.encoding import python_2_unicode_compatible
 
 
-def ensure_default_manager(sender, **kwargs):
+def ensure_default_manager(cls):
     """
     Ensures that a Model subclass contains a default manager  and sets the
     _default_manager attribute on the class. Also sets up the _base_manager
     points to a plain Manager instance (which could be the same as
     _default_manager if it's not a subclass of Manager).
     """
-    cls = sender
     if cls._meta.abstract:
         setattr(cls, 'objects', AbstractManagerDescriptor(cls))
         return
@@ -48,8 +46,6 @@ def ensure_default_manager(sender, **kwargs):
                     return
             raise AssertionError("Should never get here. Please report a bug, including your model and model manager setup.")
 
-signals.class_prepared.connect(ensure_default_manager)
-
 
 @python_2_unicode_compatible
 class BaseManager(object):