Fixed #16865 -- Made get_or_create use read database for initial get query.

Thanks Rick van Hattem for the report and trbs for the patch.

django/db/models/query.py

@@ -455,9 +455,9 @@ class QuerySet(object):
             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/releases/1.5.txt

@@ -338,6 +338,15 @@ Miscellaneous
   needs. The new default value is `0666` (octal) and the current umask value
   is first masked out.
 
+* In a multi-database situation, ``get_or_create()`` will now use a read
+  database for the initial ``get`` attempt (previously, it used only the write
+  database for all queries). This change reduces load on the write (master)
+  database, in exchange for slightly more frequent false-negatives on the
+  initial ``get`` due to replication lag. In those cases the subsequent insert
+  will still go to the master and fail, after which the existing object will be
+  fetched from the master.
+
+
 Features deprecated in 1.5
 ==========================
 

tests/modeltests/get_or_create/tests.py

@@ -64,3 +64,27 @@ class GetOrCreateTests(TestCase):
             formatted_traceback = traceback.format_exc()
             self.assertIn('obj.save', formatted_traceback)
 
+
+    def test_initial_get_on_read_db(self):
+        """
+        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. Refs #16865.
+
+        """
+        qs = Person.objects.get_query_set()
+        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)
+
+        qs = Person.objects.get_query_set()
+        p, created = qs.get_or_create(
+            first_name="Stuart", last_name="Sutcliffe")
+        self.assertFalse(created)
+        self.assertFalse(qs._for_write)