```
There’s not much that can be done with that.
Here are the calls that trigger the issue:
```plaintext
lib/topic_creator.rb:237:in `save_topic'
lib/topic_creator.rb:58:in `create'
lib/post_creator.rb:490:in `create_topic'
lib/post_creator.rb:190:in `block in create'
lib/post_creator.rb:390:in `block in transaction'
lib/post_creator.rb:390:in `transaction'
lib/post_creator.rb:188:in `create'
lib/new_post_manager.rb:318:in `perform_create_post'
lib/new_post_manager.rb:252:in `perform'
app/controllers/posts_controller.rb:210:in `block in create'
lib/distributed_memoizer.rb:16:in `block in memoize'
lib/distributed_mutex.rb:53:in `block in synchronize'
lib/distributed_mutex.rb:49:in `synchronize'
lib/distributed_mutex.rb:49:in `synchronize'
lib/distributed_mutex.rb:34:in `synchronize'
lib/distributed_memoizer.rb:12:in `memoize'
app/controllers/posts_controller.rb:209:in `create'
```
I _think_ an error needs to be raised in `TopicCreator#create`, rescued from the transaction block in `PostCreator#create`, then handled in the posts controller.
The ideal solution from my point of view would be to return an error that indicated which topic was using the external id.
Raise an error in `TopicCreator#create`:
```ruby
def create
topic = Topic.new(setup_topic_params)
if topic.external_id.present? && Topic.with_deleted.exists?(external_id: topic.external_id)
existing_topic = Topic.with_deleted.find_by(external_id: topic.external_id)
raise DuplicateExternalIdError.new(existing_topic), "External ID must be unique. Existing topic ID: #{existing_topic.id}"
end
#...
# There's probably an existing error class that could handle this, but sticking this at the bottom of the topic_creator class works for me:
class DuplicateExternalIdError < StandardError
attr_reader :topic
def initialize(topic)
@topic = topic
end
end
```
Rescue from it in `PostCreator#create`:
```ruby
def create
if valid?
begin
transaction do
build_post_stats
create_topic # this is where the error is happening
# ...
end
rescue TopicCreator::DuplicateExternalIdError => e
@errors.add(:base, "External ID must be unique. Existing topic ID: #{e.topic.id}")
end
end
```
Handle it in the posts controller:
```ruby
# posts_controller.rb
def handle_duplicate_external_id_error(exception)
render json: { error: "External ID must be unique. Existing topic ID: #{exception.topic.id}" }, status: :unprocessable_entity
end
```
That’s maybe too specific to my use case to be the right solution. It’s related to the problem with external ids and duplicate topics that’s mentioned here: [API topic's external\_ID can't be reused after deleting a topic and creating a new one - #2 by simon](https://meta.discourse.org/t/api-topics-external-id-cant-be-reused-after-deleting-a-topic-and-creating-a-new-one/280835/2). Ideally, an application that’s posting to the Discourse API could use response message to undelete and update the topic that was already using the external id.
---
### Author:  [@sam](https://meta.discourse.org/u/sam)
#### Post date: [11 ביוני, 2024, 3:33am UTC](https://meta.discourse.org/t/unhandled-pg-uniqueviolation-for-topic-external-id-field/311355/2 "2024-06-11T03:33:04Z")
Yes, happy with a PR to improve this error message!
---
### Author:  [@simon](https://meta.discourse.org/u/simon)
#### Post date: [11 ביוני, 2024, 3:49am UTC](https://meta.discourse.org/t/unhandled-pg-uniqueviolation-for-topic-external-id-field/311355/3 "2024-06-11T03:49:21Z")
Great! I’ll see what I can do.
It probably doesn’t need to be as specific as what I was proposing yesterday:
```ruby
rescue TopicCreator::DuplicateExternalIdError => e
@errors.add(:base, "External ID must be unique. Existing topic ID: #{e.topic.id}")
```
Just getting a response with details about the what constraint was violated should be good enough.