FIX: Update topic/post counter correctly when category has zero topics (#8600)

Previously, categories without any topics were being excluded from the UPDATE query. This means the counter gets stuck, and the category cannot be deleted. This change ensures that the counters get correctly set to zero.

app/models/category.rb

@@ -188,11 +188,16 @@ class Category < ActiveRecord::Base
 
     DB.exec <<~SQL
       UPDATE categories c
-         SET topic_count = x.topic_count,
+         SET topic_count = COALESCE(x.topic_count, 0),
-             post_count = x.post_count
+             post_count = COALESCE(x.post_count, 0)
-        FROM (#{topics_with_post_count}) x
+        FROM (
+              SELECT ccc.id as category_id, stats.topic_count, stats.post_count
+              FROM categories ccc
+              LEFT JOIN (#{topics_with_post_count}) stats
+              ON stats.category_id = ccc.id
+             ) x
        WHERE x.category_id = c.id
-         AND (c.topic_count <> x.topic_count OR c.post_count <> x.post_count)
+         AND (c.topic_count <> COALESCE(x.topic_count, 0) OR c.post_count <> COALESCE(x.post_count, 0))
     SQL
 
     # Yes, there are a lot of queries happening below.

spec/models/category_spec.rb

@@ -642,6 +642,22 @@ describe Category do
         expect(@uncategorized.posts_week).to eq(1)
       end
     end
+
+    context 'when there are no topics left' do
+      let!(:topic) { create_post(user: @category.user, category: @category.id).reload.topic }
+
+      it 'can update the topic count to zero' do
+        @category.reload
+        expect(@category.topic_count).to eq(1)
+        expect(@category.topics.count).to eq(2)
+        topic.delete # Delete so the post trash/destroy hook doesn't fire
+
+        Category.update_stats
+        @category.reload
+        expect(@category.topics.count).to eq(1)
+        expect(@category.topic_count).to eq(0)
+      end
+    end
   end
 
   describe "#url" do