Fixed #16905 -- Added extensible checks (nee validation) framework

This is the result of Christopher Medrela's 2013 Summer of Code project.

Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.

Also: Fixes #8579, fixes #3055, fixes #19844.

tests/sites_framework/tests.py

@@ -1,9 +1,13 @@
+from django.apps import apps
 from django.conf import settings
+from django.contrib.sites.managers import CurrentSiteManager
 from django.contrib.sites.models import Site
+from django.core import checks
+from django.db import models
 from django.test import TestCase
 
 from .models import (SyndicatedArticle, ExclusiveArticle, CustomArticle,
-    InvalidArticle, ConfusedArticle)
+    AbstractArticle)
 
 
 class SitesFrameworkTestCase(TestCase):
@@ -11,6 +15,13 @@ class SitesFrameworkTestCase(TestCase):
         Site.objects.get_or_create(id=settings.SITE_ID, domain="example.com", name="example.com")
         Site.objects.create(id=settings.SITE_ID + 1, domain="example2.com", name="example2.com")
 
+        self._old_models = apps.app_configs['sites_framework'].models.copy()
+
+    def tearDown(self):
+        apps.app_configs['sites_framework'].models = self._old_models
+        apps.all_models['sites_framework'] = self._old_models
+        apps.clear_cache()
+
     def test_site_fk(self):
         article = ExclusiveArticle.objects.create(title="Breaking News!", site_id=settings.SITE_ID)
         self.assertEqual(ExclusiveArticle.on_site.all().get(), article)
@@ -28,9 +39,38 @@ class SitesFrameworkTestCase(TestCase):
         self.assertEqual(CustomArticle.on_site.all().get(), article)
 
     def test_invalid_name(self):
-        InvalidArticle.objects.create(title="Bad News!", site_id=settings.SITE_ID)
-        self.assertRaises(ValueError, InvalidArticle.on_site.all)
+
+        class InvalidArticle(AbstractArticle):
+            site = models.ForeignKey(Site)
+
+            objects = models.Manager()
+            on_site = CurrentSiteManager("places_this_article_should_appear")
+
+        errors = InvalidArticle.check()
+        expected = [
+            checks.Error(
+                ("CurrentSiteManager could not find a field named "
+                 "'places_this_article_should_appear'."),
+                hint=('Ensure that you did not misspell the field name. '
+                      'Does the field exist?'),
+                obj=InvalidArticle.on_site,
+                id='sites.E001',
+            )
+        ]
+        self.assertEqual(errors, expected)
 
     def test_invalid_field_type(self):
-        ConfusedArticle.objects.create(title="More Bad News!", site=settings.SITE_ID)
-        self.assertRaises(TypeError, ConfusedArticle.on_site.all)
+
+        class ConfusedArticle(AbstractArticle):
+            site = models.IntegerField()
+
+        errors = ConfusedArticle.check()
+        expected = [
+            checks.Error(
+                "CurrentSiteManager requires that 'ConfusedArticle.site' must be a ForeignKey or ManyToManyField.",
+                hint=None,
+                obj=ConfusedArticle.on_site,
+                id='sites.E002',
+            )
+        ]
+        self.assertEqual(errors, expected)