Fixed #19414 -- Added admin registration decorator

Thanks stavros for the suggestion.

tests/admin_registration/tests.py

@@ -1,16 +1,23 @@
 from __future__ import unicode_literals
 
 from django.contrib import admin
+from django.contrib.admin.decorators import register
+from django.contrib.admin.sites import site
 from django.core.exceptions import ImproperlyConfigured
 from django.test import TestCase
 
-from .models import Person, Place, Location
+from .models import Person, Place, Location, Traveler
 
 
 class NameAdmin(admin.ModelAdmin):
     list_display = ['name']
     save_on_top = True
 
+
+class CustomSite(admin.AdminSite):
+    pass
+
+
 class TestRegistration(TestCase):
     def setUp(self):
         self.site = admin.AdminSite()
@@ -62,3 +69,55 @@ class TestRegistration(TestCase):
         Refs #12004.
         """
         self.assertRaises(ImproperlyConfigured, self.site.register, Location)
+
+
+class TestRegistrationDecorator(TestCase):
+    """
+    Tests the register decorator in admin.decorators
+
+    For clarity:
+
+        @register(Person)
+        class AuthorAdmin(ModelAdmin):
+            pass
+
+    is functionally equal to (the way it is written in these tests):
+
+        AuthorAdmin = register(Person)(AuthorAdmin)
+    """
+    def setUp(self):
+        self.default_site = site
+        self.custom_site = CustomSite()
+
+    def test_basic_registration(self):
+        register(Person)(NameAdmin)
+        self.assertTrue(
+            isinstance(self.default_site._registry[Person],
+                       admin.options.ModelAdmin)
+        )
+
+    def test_custom_site_registration(self):
+        register(Person, site=self.custom_site)(NameAdmin)
+        self.assertTrue(
+            isinstance(self.custom_site._registry[Person],
+                       admin.options.ModelAdmin)
+        )
+
+    def test_multiple_registration(self):
+        register(Traveler, Place)(NameAdmin)
+        self.assertTrue(
+            isinstance(self.default_site._registry[Traveler],
+                       admin.options.ModelAdmin)
+        )
+        self.assertTrue(
+            isinstance(self.default_site._registry[Place],
+                       admin.options.ModelAdmin)
+        )
+
+    def test_wrapped_class_not_a_model_admin(self):
+        self.assertRaisesMessage(ValueError, 'Wrapped class must sublcass ModelAdmin.',
+            register(Person), CustomSite)
+
+    def test_custom_site_not_an_admin_site(self):
+        self.assertRaisesMessage(ValueError, 'site must subclass AdminSite',
+            register(Person, site=Traveler), NameAdmin)