FIX: deleting category background images sometimes has no effect

app/models/category.rb

@@ -108,10 +108,6 @@ class Category < ActiveRecord::Base
     Category.reset_topic_ids_cache
   end
 
-  def self.last_updated_at
-    order('updated_at desc').limit(1).pluck(:updated_at).first.to_i
-  end
-
   def self.scoped_to_permissions(guardian, permission_types)
     if guardian.try(:is_admin?)
       all

lib/stylesheet/manager.rb

@@ -257,7 +257,7 @@ class Stylesheet::Manager
   def color_scheme_digest
 
     cs = theme&.color_scheme
-    category_updated = Category.where("uploaded_background_id IS NOT NULL").last_updated_at
+    category_updated = Category.where("uploaded_background_id IS NOT NULL").pluck(:updated_at).map(&:to_i).sum
 
     if cs || category_updated > 0
       Digest::SHA1.hexdigest "#{RailsMultisite::ConnectionManagement.current_db}-#{cs&.id}-#{cs&.version}-#{Stylesheet::Manager.last_file_updated}-#{category_updated}"

spec/components/stylesheet/manager_spec.rb

@@ -64,4 +64,30 @@ describe Stylesheet::Manager do
     # our theme better have a name with the theme_id as part of it
     expect(new_link).to include("/stylesheets/desktop_theme_#{theme.id}_")
   end
+
+  describe 'color_scheme_digest' do
+    it "changes with category background image" do
+      theme = Theme.new(
+        name: 'parent',
+        user_id: -1
+      )
+      category1 = Fabricate(:category, uploaded_background_id: 123, updated_at: 1.week.ago)
+      category2 = Fabricate(:category, uploaded_background_id: 456, updated_at: 2.days.ago)
+
+      manager = Stylesheet::Manager.new(:desktop_theme, theme.key)
+
+      digest1 = manager.color_scheme_digest
+
+      category2.update_attributes(uploaded_background_id: 789, updated_at: 1.day.ago)
+
+      digest2 = manager.color_scheme_digest
+      expect(digest2).to_not eq(digest1)
+
+      category1.update_attributes(uploaded_background_id: nil, updated_at: 5.minutes.ago)
+
+      digest3 = manager.color_scheme_digest
+      expect(digest3).to_not eq(digest2)
+      expect(digest3).to_not eq(digest1)
+    end
+  end
 end

spec/models/category_spec.rb

@@ -19,14 +19,6 @@ describe Category do
     expect(cats.errors[:name]).to be_present
   end
 
-  describe "last_updated_at" do
-    it "returns a number value of when the category was last updated" do
-      last = Category.last_updated_at
-      expect(last).to be_present
-      expect(last.to_i).to eq(last)
-    end
-  end
-
   describe "resolve_permissions" do
     it "can determine read_restricted" do
       read_restricted, resolved = Category.resolve_permissions(everyone: :full)