'
>
> ```
**Reproducible steps** :
On `3.5.0.beta8-dev` with `Discourse Calendar (and Event) - 0.5 0162ed5` enabled:
1. Create a new topic `topic 1 - that will be deleted` with a single post as `user1` who is `tl1`
2. Create new topic `topic 2 - that will break` with a single post ensure `Calendar Event` is part of this post.
3. As `user1` delete your post in `topic 1 - that will be deleted` locking the topic and replacing your post with “(topic deleted by author)”
4. As `user2` who has `moderator` permissions:
5. Select posts in `topic 1 - that will be deleted` and select the first (now deleted) post \> move to \> `topic 2 - that will break` with `preserve chronological order after merging` checked.
6. As `user2` delete the newly merged first post in `topic 2 - that will break` (the post that was just merged that reads “(topic deleted by author)”) this will make `topic 2 - that will break` break and throw the above error.
A workaround - disable the #calendar-and-event plugin. This will enable you to visit the broken topic and undelete the first post. Then re-enable the plugin.
**Discussion:**
This also identified a slightly less severe bug with Discourse where performing the above with the #calendar-and-event plugin disabled will make the topic unlisted - see separate bug report:
> [@Topic unlisted to all users including admins after merging topics](https://meta.discourse.org/t/topic-unlisted-to-all-users-including-admins-after-merging-topics/371847):
>
> Priority/Severity: Low Priority / Medium Severity Platform: 3.5.0.beta8-dev Description: After merging topics and deleting a merged post the topic efectively becomes unlisted without being marked as such and doesn’t show up even to tl4 users or Admins. It is expected that the topic will function normally and not become effectively unlisted. Speculating - It may also be the case that this topic is now viewed as “deleted” and potentially be removed automatically as part of cleanup. Reproducib…
---
### Author:  [@sam](https://meta.discourse.org/u/sam)
#### Post date: [June 26, 2025, 10:36pm UTC](https://meta.discourse.org/t/nomethoderror-in-topicscontroller-show-error-after-merging-topics/371843/2 "2025-06-26T22:36:37Z")
Thanks heaps for the detailed write up. Agree we should clean this up, going to put a #pr-welcome on this for now.
---
### Author:  [@Michael12](https://meta.discourse.org/u/Michael12)
#### Post date: [June 27, 2025, 3:40am UTC](https://meta.discourse.org/t/nomethoderror-in-topicscontroller-show-error-after-merging-topics/371843/3 "2025-06-27T03:40:43Z")
Thanks both for the detailed report and the follow-up. I just ran into a similar issue after merging topics with calendar events and couldn’t figure out why the topic was breaking. Glad to see this is documented.
I’ll keep an eye on any PRs for this. In the meantime, disabling the plugin temporarily as a workaround is helpful to know. Let me know if there’s anything I can help test.
---
### Author:  [@JammyDodger](https://meta.discourse.org/u/JammyDodger)
#### Post date: [June 27, 2025, 6:18am UTC](https://meta.discourse.org/t/nomethoderror-in-topicscontroller-show-error-after-merging-topics/371843/4 "2025-06-27T06:18:49Z")
FWIW I think I can simplify the repro:
- Enable both `Calendar enabled` and `Discourse post event enabled`
- As admin, create Topic1
- As admin, create Topic2 and include a post event using ‘Create event’ from the circle-with-a-plus composer menu
- As admin, go to Topic1 and use the topic wrench to select the OP and merge into Topic2 with ‘preserve chronological order after merging` checked
At this stage the post event is now no longer the OP of the topic for Topic2
- As admin, delete Topic2 using either the post delete of the OP or the topic delete option
Result: borkity bork. Deleted topic inaccessible. Error in `/logs` of `NoMethodError (undefined method ‘category_id’ for nil)`
* * *
Would the fix here be to prevent the merge that would replace the event as the OP?
When trying to merge an event post (one correctly placed in an OP) into another topic it’s blocked with this in the logs:
`Failed to process hijacked response correctly : ActiveRecord::RecordNotSaved : An event can only be in the first post of a topic`
(The UI error is more generic, ‘There was an error moving posts.`, which would be nice if it was more verbose)
Could something similar be applied to prevent a merge moving the event out of the OP position?
---
### Author:  [@tannerabread](https://meta.discourse.org/u/tannerabread)
#### Post date: [July 2, 2026, 6:31pm UTC](https://meta.discourse.org/t/nomethoderror-in-topicscontroller-show-error-after-merging-topics/371843/5 "2026-07-02T18:31:26Z")
>
>
> 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
---
### Author:  [@system](https://meta.discourse.org/u/system)
#### Post date: [July 4, 2026, 6:45pm UTC](https://meta.discourse.org/t/nomethoderror-in-topicscontroller-show-error-after-merging-topics/371843/6 "2026-07-04T18:45:53Z")