Ticket #16865: patch_16865_get_or_create_move_for_write-3.diff

django/db/models/query.py

@@ -455 +455 @@
             if f.attname in lookup:
                 lookup[f.name] = lookup.pop(f.attname)
         try:
-            self._for_write = True
             return self.get(**lookup), False
         except self.model.DoesNotExist:
+            self._for_write = True
             try:
                 params = dict([(k, v) for k, v in kwargs.items() if '__' not in k])
                 params.update(defaults)

docs/ref/models/querysets.txt

@@ -1318 +1318 @@
         obj = Person(first_name='John', last_name='Lennon', birthday=date(1940, 10, 9))
         obj.save()
 
+.. note::
+    When using multiple databases with some type of master-slave router ``get_or_create()``
+    will potentially use a slave for the initial lookup. If there is a lot of replication
+    lag the object might exist on the master but is not yet propagated to the slave. In
+    this case creating the object will fail and the existing object will be fetched from
+    the master database.
+
+    Changed in Django 1.5: In Django versions before 1.5 the master database was always
+    used and ``get_or_create()`` was not able to use the slaves.
+
 This pattern gets quite unwieldy as the number of fields in a model goes up.
 The above example can be rewritten using ``get_or_create()`` like so::
 

tests/modeltests/get_or_create/tests.py

@@ -64 +64 @@
             formatted_traceback = traceback.format_exc()
             self.assertIn('obj.save', formatted_traceback)
 
+        # get_or_create should only set _for_write when it's actually doing a
+        # create action. This makes sure that the initial .get() will be able
+        # to use a slave database. Specially when some form of database pinning
+        # is in place this will help to not put all the SELECT queries on the master.
+        # See #16865
+        qs = Person.objects.get_query_set()
+        p, created = qs.get_or_create(first_name="John", last_name="Lennon")
+        self.assertFalse(created)
+        self.assertFalse(qs._for_write)
+
+        p, created = qs.get_or_create(
+            first_name="Stuart", last_name="Sutcliffe", defaults={
+                'birthday': date(1940, 6, 23)
+            }
+        )
+        self.assertTrue(created)
+        self.assertTrue(qs._for_write)
+