From 5177aef37d87ea694053ecd9ddea41b471184dd4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lo=C3=AFc=20Guitaut?= <loic@discourse.org>
Date: Tue, 22 Oct 2024 15:29:07 +0200
Subject: [PATCH] =?UTF-8?q?FIX:=20Don=E2=80=99t=20try=20to=20serialize=20a?=
 =?UTF-8?q?ssociations=20in=20`PostRevisionSerializer`?=
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Currently, if an association is added as a tracked field in
`PostRevisor`, the `PostRevisionSerializer` class will try to serialize
it somehow. This will raise an error as ActiveRecord collection proxies
can't be serialized.

This patch addresses this issue by skipping any association tracked by
the `PostRevisor` class.
---
 app/serializers/post_revision_serializer.rb      |  9 +++++++--
 .../serializers/post_revision_serializer_spec.rb | 16 ++++++++++++++++
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/app/serializers/post_revision_serializer.rb b/app/serializers/post_revision_serializer.rb
index 5a6939dfa49..5c59e82d9b9 100644
--- a/app/serializers/post_revision_serializer.rb
+++ b/app/serializers/post_revision_serializer.rb
@@ -219,8 +219,13 @@ class PostRevisionSerializer < ApplicationSerializer
 
     # Retrieve any `tracked_topic_fields`
     PostRevisor.tracked_topic_fields.each_key do |field|
-      next if field == :tags
-      latest_modifications[field.to_s] = [topic.public_send(field)] if topic.respond_to?(field)
+      next unless topic.respond_to?(field)
+      topic
+        .public_send(field)
+        .then do |value|
+          next if value.try(:proxy_association)
+          latest_modifications[field.to_s] = [value]
+        end
     end
 
     latest_modifications["featured_link"] = [
diff --git a/spec/serializers/post_revision_serializer_spec.rb b/spec/serializers/post_revision_serializer_spec.rb
index bea000255f5..c70cf7f2a22 100644
--- a/spec/serializers/post_revision_serializer_spec.rb
+++ b/spec/serializers/post_revision_serializer_spec.rb
@@ -127,4 +127,20 @@ RSpec.describe PostRevisionSerializer do
       expect(json[:tags_changes]).to_not be_present
     end
   end
+
+  context "when some tracked topic fields are associations" do
+    let(:serializer) { described_class.new(post_revision, scope: guardian, root: false) }
+    let(:post_revision) { Fabricate(:post_revision, post:) }
+    let(:guardian) { Discourse.system_user.guardian }
+
+    before do
+      allow(PostRevisor).to receive(:tracked_topic_fields).and_wrap_original do |original_method|
+        original_method.call.merge(allowed_users: -> {}, allowed_groups: -> {})
+      end
+    end
+
+    it "skips them" do
+      expect { serializer.as_json }.not_to raise_error
+    end
+  end
 end