FIX: TopicQuery doesn't react well to subcategories without definitions

Also:

Move includes call higher which makes it possible to run all of the
intermediate queries for easier debugging.

Add tests for TagsController with categories in the path.

lib/topic_query.rb

@@ -663,7 +663,7 @@ class TopicQuery
     options[:visible] = false if @user && @user.id == options[:filtered_to_user]
 
     # Start with a list of all topics
-    result = Topic.unscoped
+    result = Topic.unscoped.includes(:category)
 
     if @user
       result = result.joins("LEFT OUTER JOIN topic_users AS tu ON (topics.id = tu.topic_id AND tu.user_id = #{@user.id.to_i})")
@@ -683,7 +683,7 @@ class TopicQuery
               SELECT :category_id
             ) AND
             topics.id NOT IN (
-              SELECT c3.topic_id FROM categories c3 WHERE c3.parent_category_id = :category_id
+              SELECT c3.topic_id FROM categories c3 WHERE c3.parent_category_id = :category_id AND c3.topic_id IS NOT NULL
             )
           SQL
         result = result.where(sql, category_id: category_id)
@@ -753,7 +753,7 @@ class TopicQuery
     end
 
     result = apply_ordering(result, options)
-    result = result.listable_topics.includes(:category)
+    result = result.listable_topics
 
     if options[:exclude_category_ids] && options[:exclude_category_ids].is_a?(Array) && options[:exclude_category_ids].size > 0
       result = result.where("categories.id NOT IN (?)", options[:exclude_category_ids].map(&:to_i)).references(:categories)

spec/requests/tags_controller_spec.rb

@@ -90,6 +90,72 @@ describe TagsController do
       get "/tags/test"
       expect(response.status).to eq(200)
     end
+
+    context "with a category in the path" do
+      fab!(:topic_in_category) {
+        Fabricate(
+          :topic,
+          tags: [tag],
+          category: category
+        )
+      }
+
+      fab!(:topic_in_category_without_tag) {
+        Fabricate(
+          :topic,
+          category: category
+        )
+      }
+
+      fab!(:topic_out_of_category) {
+        Fabricate(
+          :topic,
+          tags: [tag]
+        )
+      }
+
+      it "should produce the topic inside the category and not the topic outside of it" do
+        get "/tags/c/#{category.slug}/#{tag.name}.json"
+
+        topic_ids = json['topic_list']['topics'].map { |x| x['id'] }
+        expect(topic_ids).to include(topic_in_category.id)
+        expect(topic_ids).to_not include(topic_out_of_category.id)
+        expect(topic_ids).to_not include(topic_in_category_without_tag.id)
+      end
+    end
+
+    context "with a subcategory in the path" do
+      fab!(:topic_in_subcategory) {
+        Fabricate(
+          :topic,
+          tags: [tag],
+          category: subcategory
+        )
+      }
+
+      fab!(:topic_in_subcategory_without_tag) {
+        Fabricate(
+          :topic,
+          category: subcategory
+        )
+      }
+
+      fab!(:topic_out_of_subcategory) {
+        Fabricate(
+          :topic,
+          tags: [tag]
+        )
+      }
+
+      it "should produce the topic inside the subcategory and not the topic outside of it" do
+        get "/tags/c/#{category.slug}/#{subcategory.slug}/#{tag.name}.json"
+
+        topic_ids = json['topic_list']['topics'].map { |x| x['id'] }
+        expect(topic_ids).to include(topic_in_subcategory.id)
+        expect(topic_ids).to_not include(topic_out_of_subcategory.id)
+        expect(topic_ids).to_not include(topic_in_subcategory_without_tag.id)
+      end
+    end
   end
 
   describe '#check_hashtag' do