Fixed #19082 -- Enabled admin field pre-population for existing objects.

Thanks to msaelices and d1ffuz0r for the initial patch and tests.

tests/admin_views/models.py

@@ -644,8 +644,8 @@ class MainPrepopulated(models.Model):
         max_length=20,
         choices=(('option one', 'Option One'),
                  ('option two', 'Option Two')))
-    slug1 = models.SlugField()
-    slug2 = models.SlugField()
+    slug1 = models.SlugField(blank=True)
+    slug2 = models.SlugField(blank=True)
 
 class RelatedPrepopulated(models.Model):
     parent = models.ForeignKey(MainPrepopulated)

tests/admin_views/tests.py

@@ -3438,6 +3438,50 @@ class SeleniumAdminViewsFirefoxTests(AdminSeleniumWebDriverTestCase):
             slug2='option-one-tabular-inline-ignored-characters',
         )
 
+    def test_populate_existing_object(self):
+        """
+        Ensure that the prepopulation works for existing objects too, as long
+        as the original field is empty.
+        Refs #19082.
+        """
+        # Slugs are empty to start with.
+        item = MainPrepopulated.objects.create(
+            name=' this is the mAin nÀMë',
+            pubdate='2012-02-18',
+            status='option two',
+            slug1='',
+            slug2='',
+        )
+        self.admin_login(username='super',
+                         password='secret',
+                         login_url='/test_admin/admin/')
+
+        object_url = '%s%s' % (
+            self.live_server_url,
+            '/test_admin/admin/admin_views/mainprepopulated/{}/'.format(item.id))
+
+        self.selenium.get(object_url)
+        self.selenium.find_element_by_css_selector('#id_name').send_keys(' the best')
+
+        # The slugs got prepopulated since they were originally empty
+        slug1 = self.selenium.find_element_by_css_selector('#id_slug1').get_attribute('value')
+        slug2 = self.selenium.find_element_by_css_selector('#id_slug2').get_attribute('value')
+        self.assertEqual(slug1, 'main-name-best-2012-02-18')
+        self.assertEqual(slug2, 'option-two-main-name-best')
+
+        # Save the object
+        self.selenium.find_element_by_xpath('//input[@value="Save"]').click()
+        self.wait_page_loaded()
+
+        self.selenium.get(object_url)
+        self.selenium.find_element_by_css_selector('#id_name').send_keys(' hello')
+
+        # The slugs got prepopulated didn't change since they were originally not empty
+        slug1 = self.selenium.find_element_by_css_selector('#id_slug1').get_attribute('value')
+        slug2 = self.selenium.find_element_by_css_selector('#id_slug2').get_attribute('value')
+        self.assertEqual(slug1, 'main-name-best-2012-02-18')
+        self.assertEqual(slug2, 'option-two-main-name-best')
+
     def test_collapsible_fieldset(self):
         """
         Test that the 'collapse' class in fieldsets definition allows to