FIX: do not show tags with 0 count on /tags page

app/controllers/tags_controller.rb

@@ -37,14 +37,14 @@ class TagsController < ::ApplicationController
             { id: tag_group.id, name: tag_group.name, tags: self.class.tag_counts_json(tag_group.tags) }
           end
 
-          ungrouped_tags = Tag.where("tags.id NOT IN (select tag_id from tag_group_memberships)")
+          ungrouped_tags = Tag.where("tags.id NOT IN (select tag_id from tag_group_memberships) AND tags.topic_count > 0")
 
           render json: {
             tags: self.class.tag_counts_json(ungrouped_tags), # tags that don't belong to a group
             extras: { tag_groups: grouped_tag_counts }
           }
         else
-          unrestricted_tags = Tag.where("tags.id NOT IN (select tag_id from category_tags)")
+          unrestricted_tags = Tag.where("tags.id NOT IN (select tag_id from category_tags) AND tags.topic_count > 0")
 
           categories = Category.where("id in (select category_id from category_tags)")
             .where("id in (?)", guardian.allowed_category_ids)

spec/requests/tags_controller_spec.rb

@@ -5,6 +5,42 @@ describe TagsController do
     SiteSetting.tagging_enabled = true
   end
 
+  describe '#index' do
+
+    before do
+      tag = Fabricate(:tag, name: 'test')
+      topic_tag = Fabricate(:tag, name: 'topic-test', topic_count: 1)
+    end
+
+    shared_examples "successfully retrieve tags with topic_count > 0" do
+      it "should return the right response" do
+        get "/tags.json"
+
+        expect(response).to be_success
+
+        tags = JSON.parse(response.body)["tags"]
+        expect(tags.length).to eq(1)
+        expect(tags[0]['text']).to eq("topic-test")
+      end
+    end
+
+    context "with tags_listed_by_group enabled" do
+      before do
+        SiteSetting.tags_listed_by_group = true
+      end
+
+      include_examples "successfully retrieve tags with topic_count > 0"
+    end
+
+    context "with tags_listed_by_group disabled" do
+      before do
+        SiteSetting.tags_listed_by_group = false
+      end
+
+      include_examples "successfully retrieve tags with topic_count > 0"
+    end
+  end
+
   describe '#check_hashtag' do
     let(:tag) { Fabricate(:tag, name: 'test') }