Fixed #21391 -- Allow model signals to lazily reference their senders.

2 files changed, 88 insertions, 10 deletions

diff --git a/django/core/management/validation.py b/django/core/management/validation.py
index 0a976d72a1..5bf9413c20 100644
--- a/django/core/management/validation.py
+++ b/django/core/management/validation.py
@@ -1,5 +1,6 @@
 import collections
 import sys
+import types
 
 from django.conf import settings
 from django.core.management.color import color_style
@@ -25,7 +26,7 @@ def get_validation_errors(outfile, app=None):
     validates all models of all installed apps. Writes errors, if any, to outfile.
     Returns number of errors.
     """
-    from django.db import models, connection
+    from django.db import connection, models
     from django.db.models.loading import get_app_errors
     from django.db.models.deletion import SET_NULL, SET_DEFAULT
 
@@ -363,6 +364,8 @@ def get_validation_errors(outfile, app=None):
             for it in opts.index_together:
                 validate_local_fields(e, opts, "index_together", it)
 
+    validate_model_signals(e)
+
     return len(e.errors)
 
 
@@ -382,3 +385,28 @@ def validate_local_fields(e, opts, field_name, fields):
                     e.add(opts, '"%s" refers to %s. ManyToManyFields are not supported in %s.' % (field_name, f.name, field_name))
                 if f not in opts.local_fields:
                     e.add(opts, '"%s" refers to %s. This is not in the same model as the %s statement.' % (field_name, f.name, field_name))
+
+
+def validate_model_signals(e):
+    """Ensure lazily referenced model signals senders are installed."""
+    from django.db import models
+
+    for name in dir(models.signals):
+        obj = getattr(models.signals, name)
+        if isinstance(obj, models.signals.ModelSignal):
+            for reference, receivers in obj.unresolved_references.items():
+                for receiver, _, _ in receivers:
+                    # The receiver is either a function or an instance of class
+                    # defining a `__call__` method.
+                    if isinstance(receiver, types.FunctionType):
+                        description = "The `%s` function" % receiver.__name__
+                    else:
+                        description = "An instance of the `%s` class" % receiver.__class__.__name__
+                    e.add(
+                        receiver.__module__,
+                        "%s was connected to the `%s` signal "
+                        "with a lazy reference to the '%s' sender, "
+                        "which has not been installed." % (
+                            description, name, '.'.join(reference)
+                        )
+                    )
diff --git a/django/db/models/signals.py b/django/db/models/signals.py
index 6b7605839c..6b011c2099 100644
--- a/django/db/models/signals.py
+++ b/django/db/models/signals.py
@@ -1,20 +1,70 @@
+from collections import defaultdict
+
+from django.db.models.loading import get_model
 from django.dispatch import Signal
+from django.utils import six
+
 
 class_prepared = Signal(providing_args=["class"])
 
-pre_init = Signal(providing_args=["instance", "args", "kwargs"], use_caching=True)
-post_init = Signal(providing_args=["instance"], use_caching=True)
 
-pre_save = Signal(providing_args=["instance", "raw", "using", "update_fields"],
-                 use_caching=True)
-post_save = Signal(providing_args=["instance", "raw", "created", "using", "update_fields"], use_caching=True)
+class ModelSignal(Signal):
+    """
+    Signal subclass that allows the sender to be lazily specified as a string
+    of the `app_label.ModelName` form.
+    """
+
+    def __init__(self, *args, **kwargs):
+        super(ModelSignal, self).__init__(*args, **kwargs)
+        self.unresolved_references = defaultdict(list)
+        class_prepared.connect(self._resolve_references)
+
+    def _resolve_references(self, sender, **kwargs):
+        opts = sender._meta
+        reference = (opts.app_label, opts.object_name)
+        try:
+            receivers = self.unresolved_references.pop(reference)
+        except KeyError:
+            pass
+        else:
+            for receiver, weak, dispatch_uid in receivers:
+                super(ModelSignal, self).connect(
+                    receiver, sender=sender, weak=weak, dispatch_uid=dispatch_uid
+                )
+
+    def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):
+        if isinstance(sender, six.string_types):
+            try:
+                app_label, object_name = sender.split('.')
+            except ValueError:
+                raise ValueError(
+                    "Specified sender must either be a model or a "
+                    "model name of the 'app_label.ModelName' form."
+                )
+            sender = get_model(app_label, object_name, only_installed=False)
+            if sender is None:
+                reference = (app_label, object_name)
+                self.unresolved_references[reference].append(
+                    (receiver, weak, dispatch_uid)
+                )
+                return
+        super(ModelSignal, self).connect(
+            receiver, sender=sender, weak=weak, dispatch_uid=dispatch_uid
+        )
 
-pre_delete = Signal(providing_args=["instance", "using"], use_caching=True)
-post_delete = Signal(providing_args=["instance", "using"], use_caching=True)
+pre_init = ModelSignal(providing_args=["instance", "args", "kwargs"], use_caching=True)
+post_init = ModelSignal(providing_args=["instance"], use_caching=True)
+
+pre_save = ModelSignal(providing_args=["instance", "raw", "using", "update_fields"],
+                       use_caching=True)
+post_save = ModelSignal(providing_args=["instance", "raw", "created", "using", "update_fields"], use_caching=True)
+
+pre_delete = ModelSignal(providing_args=["instance", "using"], use_caching=True)
+post_delete = ModelSignal(providing_args=["instance", "using"], use_caching=True)
+
+m2m_changed = ModelSignal(providing_args=["action", "instance", "reverse", "model", "pk_set", "using"], use_caching=True)
 
 pre_migrate = Signal(providing_args=["app", "create_models", "verbosity", "interactive", "db"])
 pre_syncdb = pre_migrate
 post_migrate = Signal(providing_args=["class", "app", "created_models", "verbosity", "interactive", "db"])
 post_syncdb = post_migrate
-
-m2m_changed = Signal(providing_args=["action", "instance", "reverse", "model", "pk_set", "using"], use_caching=True)