FIX: Base topic details message on current category and tag tracking state (#12937)

The user may have changed their category or tag tracking settings since a topic was tracked/watched based on those settings in the past. In that case we need to alter the reason message we show them otherwise it is very confusing for the end user to be told they are tracking a topic because of a category, when they are no longer tracking that category.

For example: "You will see a count of new replies because you are tracking this category." becomes: "You will see a count of new replies because you were tracking this category in the past."

To do this, it was necessary to add tag and category tracking info to current user serializer. I improved the serializer code so it only does 3 SQL queries instead of 9 to get the tracking information for tags and categories for the current user.

spec/models/category_user_spec.rb

@@ -219,6 +219,54 @@ describe CategoryUser do
 
       expect(CategoryUser.where(user_id: user.id).count).to eq(0)
     end
+  end
 
+  describe "#notification_levels_for" do
+    let(:guardian) { Guardian.new(user) }
+    let!(:category1) { Fabricate(:category) }
+    let!(:category2) { Fabricate(:category) }
+    let!(:category3) { Fabricate(:category) }
+    let!(:category4) { Fabricate(:category) }
+    let!(:category5) { Fabricate(:category) }
+
+    context "for anon" do
+      let(:user) { nil }
+      before do
+        SiteSetting.default_categories_watching = category1.id.to_s
+        SiteSetting.default_categories_tracking = category2.id.to_s
+        SiteSetting.default_categories_watching_first_post = category3.id.to_s
+        SiteSetting.default_categories_regular = category4.id.to_s
+        SiteSetting.default_categories_muted = category5.id.to_s
+      end
+      it "every category from the default_categories_* site settings get overridden to regular, except for muted" do
+        levels = CategoryUser.notification_levels_for(guardian)
+        expect(levels[category1.id]).to eq(CategoryUser.notification_levels[:regular])
+        expect(levels[category2.id]).to eq(CategoryUser.notification_levels[:regular])
+        expect(levels[category3.id]).to eq(CategoryUser.notification_levels[:regular])
+        expect(levels[category4.id]).to eq(CategoryUser.notification_levels[:regular])
+        expect(levels[category5.id]).to eq(CategoryUser.notification_levels[:muted])
+      end
+    end
+
+    context "for a user" do
+      before do
+        CategoryUser.create(user: user, category: category1, notification_level: CategoryUser.notification_levels[:watching])
+        CategoryUser.create(user: user, category: category2, notification_level: CategoryUser.notification_levels[:tracking])
+        CategoryUser.create(user: user, category: category3, notification_level: CategoryUser.notification_levels[:watching_first_post])
+        CategoryUser.create(user: user, category: category4, notification_level: CategoryUser.notification_levels[:regular])
+        CategoryUser.create(user: user, category: category5, notification_level: CategoryUser.notification_levels[:muted])
+      end
+      it "gets the category_user notification levels for all categories the user is tracking and does not
+      include categories the user is not tracking at all" do
+        category6 = Fabricate(:category)
+        levels = CategoryUser.notification_levels_for(guardian)
+        expect(levels[category1.id]).to eq(CategoryUser.notification_levels[:watching])
+        expect(levels[category2.id]).to eq(CategoryUser.notification_levels[:tracking])
+        expect(levels[category3.id]).to eq(CategoryUser.notification_levels[:watching_first_post])
+        expect(levels[category4.id]).to eq(CategoryUser.notification_levels[:regular])
+        expect(levels[category5.id]).to eq(CategoryUser.notification_levels[:muted])
+        expect(levels.key?(category6.id)).to eq(false)
+      end
+    end
   end
 end