Preview Generator cron job creating errors

[iamhives]

Getting the following error when my preview generator cron job runs. Running Preview Generator 5.14.0 on Nextcloud 34.0.3

Getting log entries like this:
Reason : Error while running background job OCA\PreviewGenerator\BackgroundJob\PreviewJob (id: 37420, arguments: null)

2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00 In DbalException.php line 50:
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00 An exception occurred while executing a query: SQLSTATE[23000]: Integrity c
2026-08-19T16:39:35+00:00 onstraint violation: 1052 Column 'file_id' in where clause is ambiguous
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00 In ExceptionConverter.php line 74:
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00 An exception occurred while executing a query: SQLSTATE[23000]: Integrity c
2026-08-19T16:39:35+00:00 onstraint violation: 1052 Column 'file_id' in where clause is ambiguous
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00 In Exception.php line 24:
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00 SQLSTATE[23000]: Integrity constraint violation: 1052 Column 'file_id' in w
2026-08-19T16:39:35+00:00 here clause is ambiguous
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00 In Statement.php line 130:
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00 SQLSTATE[23000]: Integrity constraint violation: 1052 Column 'file_id' in w
2026-08-19T16:39:35+00:00 here clause is ambiguous
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00
2026-08-19T16:39:35+00:00 preview:pre-generate
2026-08-19T16:39:35+00:00

[Shr3ps]

Hello there, I got this error and launch an AI analysis, here is the result:

Root cause: it's not this app — it's Nextcloud server core

I traced it rather than guessing, and the app is clean: previewgenerator contains zero SQL joins (grep -rni join lib/ → nothing). Every query it runs touches only oc_preview_generation (PreGenerateService.php:69, PostWriteListener.php:44, QueueStats.php:80), so it cannot produce an ambiguous column.

The ambiguity is in core OC\Preview\Db\PreviewMapper, reached via IPreview::generatePreviews() → Generator::savePreview():

• joinLocation() builds previews p LEFT JOIN preview_locations l LEFT JOIN preview_versions v
• both previews and preview_versions have a file_id column
• getByFileId() and getPreviewForSpecification() passed unqualified column names → MariaDB/MySQL error 1052

Already fixed upstream: server PR #63286, backported as #63347 — commit 8dfcdac on stable34, merged 2026-08-18. I verified v34.0.3 (your version, still the latest tag today) does not have it, and stable34 does. So it lands in 34.0.4.

Interim fix for your server (verified to apply to 34.0.3)

I dry-ran the upstream patch against the real 34.0.3 file — applies cleanly (two hunks at offset −4):

curl -sSLo /tmp/nc-63286.patch https://github.com/nextcloud/server/commit/8dfcdacbe78b35958915585928239ad82ce6e0e6.patch
cd /var/www/nextcloud
cp -a lib/private/Preview/Db/PreviewMapper.php /root/PreviewMapper.php.34.0.3.bak
sudo -u www-data git apply --check --include='lib/private/Preview/Db/PreviewMapper.php' /tmp/nc-63286.patch # dry run
sudo -u www-data git apply --include='lib/private/Preview/Db/PreviewMapper.php' /tmp/nc-63286.patch
systemctl reload php8.3-fpm # clear opcache for web requests

--include is required: the patch also touches tests/lib/..., which isn't shipped in a release tarball (git apply skips it, GNU patch 2.7.6 would choke). The fix qualifies p.file_id and maps plain keys to the right alias, plus types bool/int params properly. Caveats: Settings → Overview will flag an integrity mismatch until 34.0.4 overwrites the file — that's expected.

One real app-side issue worth fixing here

PreGenerateService::startProcessing() (lib/Service/PreGenerateService.php:66) deletes the queue row inside the transaction, then calls processRow() outside it. processFile() rethrows any DbalException that isn't a unique-constraint violation — so this crash both aborted the whole run and permanently dropped that file's queue entry (same for PreviewJob). Every entry drained during the failing runs is lost, not retried.

Sources:

• Preview Generator cron job creating errors #673
• server commit 8dfcdac — backport #63347 of #63286
• help.nextcloud.com thread on the same error

[gmta]

The preview generator removing queue rows before processing is a serious issue causing previews not to be created or updated - at the very least this behavior should be configurable?