Fixed #11 -- Support Django 1.6's ATOMIC_REQUESTS

In Django 1.6 `TransactionMiddleware` was deprecated in favor of
`ATOMIC_REQUESTS`. With this setting activated, all views will be
wrapped in `transaction.atomic`. However the `SessionStore` was
saving sessions within a `transaction.commit_on_success`
wrapper. Mixing old-style and new-style is not supported in all
scenarios, so this resulted in an error.

example/settings.py

@@ -51,6 +51,7 @@
     'default': {
         'ENGINE': 'django.db.backends.sqlite3',
         'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
+        'ATOMIC_REQUESTS': True,
     }
 }
 

tests/settings.py

@@ -29,6 +29,7 @@
 DATABASES = {
     'default': {
         'ENGINE': 'django.db.backends.sqlite3',
+        'ATOMIC_REQUESTS': True,
     }
 }
 

user_sessions/backends/db.py

@@ -1,6 +1,7 @@
 import logging
-from django.contrib import auth
 
+import django
+from django.contrib import auth
 from django.contrib.sessions.backends.base import SessionBase, CreateError
 from django.core.exceptions import SuspiciousOperation
 from django.db import IntegrityError, transaction, router
@@ -74,8 +75,12 @@ def save(self, must_create=False):
         )
         using = router.db_for_write(Session, instance=obj)
         try:
-            with transaction.commit_on_success(using):
-                obj.save(force_insert=must_create, using=using)
+            if django.VERSION >= (1, 6):
+                with transaction.atomic(using):
+                    obj.save(force_insert=must_create, using=using)
+            else:
+                with transaction.commit_on_success(using):
+                    obj.save(force_insert=must_create, using=using)
         except IntegrityError as e:
             if must_create and 'session_key' in str(e):
                 raise CreateError