DEV: Fix `TopcisFilter#filter_tags` not working for a single tag (#20840)

Follow-up to dd88fdeabc

lib/topics_filter.rb

@@ -74,10 +74,10 @@ class TopicsFilter
 
   def exclude_topics_with_any_tags(tag_ids)
     @scope =
-      @scope
-        .left_joins(:topic_tags)
-        .where("topic_tags.tag_id IS NULL OR topic_tags.tag_id NOT IN (?)", tag_ids)
-        .distinct(:id)
+      @scope.where(
+        "topics.id NOT IN (SELECT DISTINCT topic_id FROM topic_tags WHERE topic_tags.tag_id IN (?))",
+        tag_ids,
+      )
   end
 
   def include_topics_with_all_tags(tag_ids)

spec/lib/topics_filter_spec.rb

@@ -153,6 +153,13 @@ RSpec.describe TopicsFilter do
     end
 
     it "should only return topics that are not tagged with all of the specified tags when `match_all` is `true` and `exclude` is `true`" do
+      expect(
+        TopicsFilter
+          .new(guardian: Guardian.new)
+          .filter_tags(tag_names: [tag.name], match_all: true, exclude: true)
+          .pluck(:id),
+      ).to contain_exactly(topic_without_tag.id, topic_with_tag2.id, topic_with_group_only_tag.id)
+
       expect(
         TopicsFilter
           .new(guardian: Guardian.new)
@@ -167,6 +174,13 @@ RSpec.describe TopicsFilter do
     end
 
     it "should only return topics that are not tagged with any of the specified tags when `match_all` is `false` and `exclude` is `true`" do
+      expect(
+        TopicsFilter
+          .new(guardian: Guardian.new)
+          .filter_tags(tag_names: [tag.name], match_all: false, exclude: true)
+          .pluck(:id),
+      ).to contain_exactly(topic_without_tag.id, topic_with_group_only_tag.id, topic_with_tag2.id)
+
       expect(
         TopicsFilter
           .new(guardian: Guardian.new)