FIX: Make topic query include topics from sub-sub-categories (#8709)

lib/topic_query.rb

@@ -677,15 +677,18 @@ class TopicQuery
       else
         sql = <<~SQL
             categories.id IN (
-              SELECT c2.id FROM categories c2 WHERE c2.parent_category_id = :category_id
-              UNION ALL
-              SELECT :category_id
-            ) AND
-            topics.id NOT IN (
-              SELECT c3.topic_id FROM categories c3 WHERE c3.parent_category_id = :category_id AND c3.topic_id IS NOT NULL
+              WITH RECURSIVE subcategories AS (
+                SELECT :category_id id, 1 depth
+                UNION
+                SELECT categories.id, (subcategories.depth + 1) depth
+                FROM categories
+                JOIN subcategories ON subcategories.id = categories.parent_category_id
+                WHERE subcategories.depth < :max_category_nesting
             )
+              SELECT subcategories.id FROM subcategories
+            ) AND (categories.id = :category_id OR topics.id != categories.topic_id)
           SQL
-        result = result.where(sql, category_id: category_id)
+        result = result.where(sql, category_id: category_id, max_category_nesting: SiteSetting.max_category_nesting)
       end
       result = result.references(:categories)
 

spec/components/topic_query_spec.rb

@@ -153,6 +153,7 @@ describe TopicQuery do
 
     context 'subcategories' do
       let!(:subcategory) { Fabricate(:category_with_definition, parent_category_id: category.id) }
+      let(:subsubcategory) { Fabricate(:category_with_definition, parent_category_id: subcategory.id) }
 
       it "works with subcategories" do
         expect(TopicQuery.new(moderator, category: category.id).list_latest.topics.size).to eq(1)
@@ -160,6 +161,23 @@ describe TopicQuery do
         expect(TopicQuery.new(moderator, category: category.id, no_subcategories: true).list_latest.topics.size).to eq(1)
       end
 
+      it "works with subsubcategories" do
+        SiteSetting.max_category_nesting = 3
+
+        Fabricate(:topic, category: category)
+        Fabricate(:topic, category: subcategory)
+        Fabricate(:topic, category: subsubcategory)
+
+        SiteSetting.max_category_nesting = 2
+        expect(TopicQuery.new(moderator, category: category.id).list_latest.topics.size).to eq(3)
+        expect(TopicQuery.new(moderator, category: subcategory.id).list_latest.topics.size).to eq(3)
+        expect(TopicQuery.new(moderator, category: subsubcategory.id).list_latest.topics.size).to eq(2)
+
+        SiteSetting.max_category_nesting = 3
+        expect(TopicQuery.new(moderator, category: category.id).list_latest.topics.size).to eq(4)
+        expect(TopicQuery.new(moderator, category: subcategory.id).list_latest.topics.size).to eq(3)
+        expect(TopicQuery.new(moderator, category: subsubcategory.id).list_latest.topics.size).to eq(2)
+      end
     end
   end