Backport of #179: Make get_available_languages use cached translations if they were prefetched. Fixes #97.

Author: spectras

docs/public/admin.rst

@@ -20,6 +20,10 @@ all_translations
     languages in which this object is available. Entries are linked to their
     corresponding admin page.
 
+    .. note:: You should add `prefetch_related('translations')` to your queryset
+              if you use this in :attr:`~django.contrib.admin.ModelAdmin.list_display`,
+              else one query will be run for every item in the list.
+
 
 ***********************************************************
 ModelAdmin APIs you should not change on TranslatableAdmin

hvad/models.py

@@ -312,8 +312,10 @@ def lazy_translation_getter(self, name, default=None):
         return getattr(translation, name, default)
 
     def get_available_languages(self):
-        manager = self._meta.translations_model.objects
-        return manager.filter(master=self).values_list('language_code', flat=True)
+        qs = getattr(self, self._meta.translations_accessor).all()
+        if qs._result_cache is not None:
+            return [obj.language_code for obj in qs]
+        return qs.values_list('language_code', flat=True)
 
     #===========================================================================
     # Internals

hvad/tests/admin.py

@@ -16,6 +16,8 @@
 from hvad.test_utils.context_managers import SettingsOverride
 from hvad.test_utils.project.app.models import Normal, SimpleRelated, Other, AutoPopulated
 
+PREFETCH_RELATED = (django.VERSION >= (1, 4))
+
 class BaseAdminTests(object):
     def _get_admin(self, model):
         return admin.site._registry[model]
@@ -83,13 +85,21 @@ def test_all_translations(self):
         obj = Normal.objects.language("en").create(
             shared_field="shared",
         )
+
+        # Get the object back from db to test the use of prefetch_related
+        qs = Normal.objects.language('en')
+        if PREFETCH_RELATED:
+            qs = qs.prefetch_related('translations')
+        obj = qs.get(pk=obj.pk)
         with LanguageOverride('en'):
-            self.assertTrue(myadmin.all_translations(obj).find("<strong>") != -1)
-            # Entries should be linked to the corresponding translation page
-            self.assertTrue(myadmin.all_translations(obj).find("?language=en") != -1)
+            with self.assertNumQueries(0 if PREFETCH_RELATED else 2):
+                self.assertTrue(myadmin.all_translations(obj).find("<strong>") != -1)
+                # Entries should be linked to the corresponding translation page
+                self.assertTrue(myadmin.all_translations(obj).find("?language=en") != -1)
 
         with LanguageOverride('ja'):
-            self.assertTrue(myadmin.all_translations(obj).find("<strong>") == -1)
+            with self.assertNumQueries(0 if PREFETCH_RELATED else 1):
+                self.assertTrue(myadmin.all_translations(obj).find("<strong>") == -1)
 
         # An unsaved object, shouldn't have any translations