Fixed #27673 -- Made admin's checks run at check time instead of during registration.

Thanks Morgan Aubert for the test.

AUTHORS

@@ -557,6 +557,7 @@ answer newbie questions, and generally made Django that much better:
     mitakummaa@gmail.com
     mmarshall
     Moayad Mardini <moayad.m@gmail.com>
+    Morgan Aubert <morgan.aubert@zoho.com>
     Moritz Sichert <moritz.sichert@googlemail.com>
     Morten Bagai <m@bagai.com>
     msaelices <msaelices@gmail.com>

django/contrib/admin/checks.py

@@ -18,10 +18,12 @@ from django.forms.models import (
 from django.template.engine import Engine
 
 
-def check_admin_app(**kwargs):
-    from django.contrib.admin.sites import system_check_errors
-
-    return system_check_errors
+def check_admin_app(app_configs, **kwargs):
+    from django.contrib.admin.sites import all_sites
+    errors = []
+    for site in all_sites:
+        errors.extend(site.check(app_configs))
+    return errors
 
 
 def check_dependencies(**kwargs):

django/contrib/admin/sites.py

@@ -1,4 +1,5 @@
 from functools import update_wrapper
+from weakref import WeakSet
 
 from django.apps import apps
 from django.conf import settings
@@ -16,7 +17,7 @@ from django.views.decorators.cache import never_cache
 from django.views.decorators.csrf import csrf_protect
 from django.views.i18n import JavaScriptCatalog
 
-system_check_errors = []
+all_sites = WeakSet()
 
 
 class AlreadyRegistered(Exception):
@@ -63,6 +64,26 @@ class AdminSite(object):
         self.name = name
         self._actions = {'delete_selected': actions.delete_selected}
         self._global_actions = self._actions.copy()
+        all_sites.add(self)
+
+    def check(self, app_configs):
+        """
+        Run the system checks on all ModelAdmins, except if they aren't
+        customized at all.
+        """
+        if not settings.DEBUG:
+            return []
+
+        if app_configs is None:
+            app_configs = apps.get_app_configs()
+        app_configs = set(app_configs)  # Speed up lookups below
+
+        errors = []
+        modeladmins = (o for o in self._registry.values() if o.__class__ is not ModelAdmin)
+        for modeladmin in modeladmins:
+            if modeladmin.model._meta.app_config in app_configs:
+                errors.extend(modeladmin.check())
+        return errors
 
     def register(self, model_or_iterable, admin_class=None, **options):
         """
@@ -105,11 +126,7 @@ class AdminSite(object):
                     admin_class = type("%sAdmin" % model.__name__, (admin_class,), options)
 
                 # Instantiate the admin class to save in the registry
-                admin_obj = admin_class(model, self)
-                if admin_class is not ModelAdmin and settings.DEBUG:
-                    system_check_errors.extend(admin_obj.check())
-
-                self._registry[model] = admin_obj
+                self._registry[model] = admin_class(model, self)
 
     def unregister(self, model_or_iterable):
         """

tests/admin_checks/tests.py

@@ -7,7 +7,9 @@ from django.contrib.contenttypes.admin import GenericStackedInline
 from django.core import checks
 from django.test import SimpleTestCase, override_settings
 
-from .models import Album, Book, City, Influence, Song, State, TwoAlbumFKAndAnE
+from .models import (
+    Album, Author, Book, City, Influence, Song, State, TwoAlbumFKAndAnE,
+)
 
 
 class SongForm(forms.ModelForm):
@@ -52,7 +54,6 @@ class SystemChecksTestCase(SimpleTestCase):
             self.assertEqual(errors, expected)
         finally:
             admin.site.unregister(Song)
-            admin.sites.system_check_errors = []
 
     @override_settings(INSTALLED_APPS=['django.contrib.admin'])
     def test_contenttypes_dependency(self):
@@ -105,7 +106,27 @@ class SystemChecksTestCase(SimpleTestCase):
             self.assertEqual(errors, expected)
         finally:
             custom_site.unregister(Song)
-            admin.sites.system_check_errors = []
+
+    @override_settings(DEBUG=True)
+    def test_allows_checks_relying_on_other_modeladmins(self):
+        class MyBookAdmin(admin.ModelAdmin):
+            def check(self, **kwargs):
+                errors = super(MyBookAdmin, self).check(**kwargs)
+                author_admin = self.admin_site._registry.get(Author)
+                if author_admin is None:
+                    errors.append('AuthorAdmin missing!')
+                return errors
+
+        class MyAuthorAdmin(admin.ModelAdmin):
+            pass
+
+        admin.site.register(Book, MyBookAdmin)
+        admin.site.register(Author, MyAuthorAdmin)
+        try:
+            self.assertEqual(admin.site.check(None), [])
+        finally:
+            admin.site.unregister(Book)
+            admin.site.unregister(Author)
 
     def test_field_name_not_in_list_display(self):
         class SongAdmin(admin.ModelAdmin):