`NoMethodError in TopicsController#show` error after merging topics

tannerabread · July 2, 2026, 6:31pm

FIX: Topic Merging with Events (#41369)

main ← fix-event-topic-merging

merged 06:30PM - 02 Jul 26 UTC

meta topic: [`/t/371843`](https://meta.discourse.org/t/nomethoderror-in-topicsco…ntroller-show-error-after-merging-topics/371843) This fixes the bug pointed out about merging topics with events. The issue was that when merging topics with events, it circumvented the event validation logic and allowed event posts to be merged into topics that already have events or displace the event post in the destination. Add a topic extension to handle the event validation logic when merging topics with events. Also add null safety in the event serializer to handle cases where the event post is no longer valid.

This has been fixed + some additional guarding for other similar scenarios

Topic		Replies	Views
Topic unlisted to all users including admins after merging topics Product Support	3	188	June 28, 2025
User merge failed, corrupted database entries due to third party plugin Product Support merge-user	31	3381	October 6, 2020
Topic in a category make unreachable the category and user activity and the topic itself Product Support	1	963	July 12, 2019
"Move to existing topic" admin feature not working [now fixed] Bug	14	1055	May 29, 2019
Can't delete this category because it has 1 topic Product Support	11	1403	February 2, 2021

`NoMethodError in TopicsController#show` error after merging topics

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