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

tests/model_validation/tests.py

@@ -1,10 +1,22 @@
 from django.core import management
+from django.core.management.validation import (
+    ModelErrorCollection, validate_model_signals
+)
+from django.db.models.signals import post_init
 from django.test import TestCase
 from django.utils import six
 
 
-class ModelValidationTest(TestCase):
+class OnPostInit(object):
+    def __call__(self, **kwargs):
+        pass
 
+
+def on_post_init(**kwargs):
+    pass
+
+
+class ModelValidationTest(TestCase):
     def test_models_validate(self):
         # All our models should validate properly
         # Validation Tests:
@@ -13,3 +25,23 @@ class ModelValidationTest(TestCase):
         #   * related_name='+' doesn't clash with another '+'
         #       See: https://code.djangoproject.com/ticket/21375
         management.call_command("validate", stdout=six.StringIO())
+
+    def test_model_signal(self):
+        unresolved_references = post_init.unresolved_references.copy()
+        post_init.connect(on_post_init, sender='missing-app.Model')
+        post_init.connect(OnPostInit(), sender='missing-app.Model')
+        e = ModelErrorCollection(six.StringIO())
+        validate_model_signals(e)
+        self.assertSetEqual(set(e.errors), {
+            ('model_validation.tests',
+                "The `on_post_init` function was connected to the `post_init` "
+                "signal with a lazy reference to the 'missing-app.Model' "
+                "sender, which has not been installed."
+            ),
+            ('model_validation.tests',
+                "An instance of the `OnPostInit` class was connected to "
+                "the `post_init` signal with a lazy reference to the "
+                "'missing-app.Model' sender, which has not been installed."
+            )
+        })
+        post_init.unresolved_references = unresolved_references