Use iterator in access token migration (#1522)

Author: acidtv

AUTHORS

@@ -122,3 +122,4 @@ Wouter Klein Heerenbrink
 Yaroslav Halchenko
 Yuri Savin
 Miriam Forner
+Alex Kerkum

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 * #1517 OP prompts for logout when no OP session
 * #1512 client_secret not marked sensitive
+* #1521 Fix 0012 migration loading access token table into memory
 <!--
 ### Security
 -->

oauth2_provider/migrations/0012_add_token_checksum.py

@@ -9,7 +9,7 @@ def forwards_func(apps, schema_editor):
     Forward migration touches every "old" accesstoken.token which will cause the checksum to be computed.
     """
     AccessToken = apps.get_model(oauth2_settings.ACCESS_TOKEN_MODEL)
-    accesstokens = AccessToken._default_manager.all()
+    accesstokens = AccessToken._default_manager.iterator()
     for accesstoken in accesstokens:
         accesstoken.save(update_fields=['token_checksum'])