FIX: 404 when filtering by category, no sub-category, and a tag

app/controllers/tags_controller.rb

@@ -197,6 +197,10 @@ class TagsController < ::ApplicationController
       slug_or_id = params[:category]
       return true if slug_or_id.nil?
 
+      if slug_or_id == 'none' && params[:parent_category]
+        @filter_on_category = Category.query_category(params[:parent_category], nil)
+        params[:no_subcategories] = 'true'
+      else
         parent_slug_or_id = params[:parent_category]
 
         parent_category_id = nil
@@ -206,6 +210,8 @@ class TagsController < ::ApplicationController
         end
 
         @filter_on_category = Category.query_category(slug_or_id, parent_category_id)
+      end
+
       redirect_or_not_found and return if !@filter_on_category
 
       guardian.ensure_can_see!(@filter_on_category)

spec/controllers/tags_controller_spec.rb

@@ -0,0 +1,42 @@
+require 'rails_helper'
+
+describe TagsController do
+  describe 'show_latest' do
+    let(:tag)         { Fabricate(:tag) }
+    let(:category)    { Fabricate(:category) }
+    let(:subcategory) { Fabricate(:category, parent_category_id: category.id) }
+
+    context 'tagging disabled' do
+      it "returns 404" do
+        xhr :get, :show_latest, tag_id: tag.name
+        expect(response.status).to eq(404)
+      end
+    end
+
+    context 'tagging enabled' do
+      before do
+        SiteSetting.tagging_enabled = true
+      end
+
+      it "can filter by tag" do
+        xhr :get, :show_latest, tag_id: tag.name
+        expect(response).to be_success
+      end
+
+      it "can filter by category and tag" do
+        xhr :get, :show_latest, tag_id: tag.name, category: category.slug
+        expect(response).to be_success
+      end
+
+      it "can filter by category, sub-category, and tag" do
+        xhr :get, :show_latest, tag_id: tag.name, category: subcategory.slug, parent_category: category.slug
+        expect(response).to be_success
+      end
+
+      it "can filter by category, no sub-category, and tag" do
+        xhr :get, :show_latest, tag_id: tag.name, category: 'none', parent_category: category.slug
+        expect(response).to be_success
+      end
+    end
+  end
+end