From 4efc126635ecca3f29b3d06983118af0ea0ad0d0 Mon Sep 17 00:00:00 2001
From: Dan Ungureanu <dan@ungureanu.me>
Date: Fri, 26 Jun 2020 22:25:38 +0300
Subject: [PATCH] FIX: Serialize an empty array if no suggested topics exist
 (#10134)

It used to return nil, which was ambiguous (empty vs absent
result).
---
 app/serializers/suggested_topics_mixin.rb     |  4 +--
 .../serializers/topic_view_serializer_spec.rb | 34 +++++++++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/app/serializers/suggested_topics_mixin.rb b/app/serializers/suggested_topics_mixin.rb
index 9fca424b0f5..74d3d65a9cf 100644
--- a/app/serializers/suggested_topics_mixin.rb
+++ b/app/serializers/suggested_topics_mixin.rb
@@ -7,11 +7,11 @@ module SuggestedTopicsMixin
   end
 
   def include_related_messages?
-    object.next_page.nil? && object.related_messages&.topics.present?
+    object.next_page.nil? && object.related_messages&.topics
   end
 
   def include_suggested_topics?
-    object.next_page.nil? && object.suggested_topics&.topics.present?
+    object.next_page.nil? && object.suggested_topics&.topics
   end
 
   def related_messages
diff --git a/spec/serializers/topic_view_serializer_spec.rb b/spec/serializers/topic_view_serializer_spec.rb
index 55a9652fddc..e28182765e4 100644
--- a/spec/serializers/topic_view_serializer_spec.rb
+++ b/spec/serializers/topic_view_serializer_spec.rb
@@ -118,6 +118,40 @@ describe TopicViewSerializer do
         expect(json[:suggested_topics]).to eq(nil)
       end
     end
+
+    describe 'with private messages' do
+      let!(:topic) do
+        Fabricate(:private_message_topic,
+          highest_post_number: 1,
+          topic_allowed_users: [
+            Fabricate.build(:topic_allowed_user, user: user)
+          ]
+        )
+      end
+
+      let!(:topic2) do
+        Fabricate(:private_message_topic,
+          highest_post_number: 1,
+          topic_allowed_users: [
+            Fabricate.build(:topic_allowed_user, user: user)
+          ]
+        )
+      end
+
+      it 'includes suggested topics' do
+        TopicUser.change(user, topic2.id, notification_level: TopicUser.notification_levels[:tracking])
+
+        json = serialize_topic(topic, user)
+        expect(json[:suggested_topics].map { |t| t[:id] }).to contain_exactly(topic2.id)
+      end
+
+      it 'does not include suggested topics if all PMs are read' do
+        TopicUser.update_last_read(user, topic2.id, 1, 1, 0)
+
+        json = serialize_topic(topic, user)
+        expect(json[:suggested_topics]).to eq([])
+      end
+    end
   end
 
   describe 'when tags added to private message topics' do