module VoluntaryRanking
  class Engine < ::Rails::Engine
    config.autoload_paths << File.expand_path("../..", __FILE__)
    config.autoload_paths << File.expand_path("../../../app/models/concerns", __FILE__)
     
    initializer 'configure ember-rails ranking', before: 'ember_rails.setup_vendor' do
      config.handlebars.templates_root << 'voluntary_ranking/templates'
    end
    
    config.to_prepare do
      User.send :include, Concerns::Model::User::Ranking
    end
  end
end
module VoluntaryRanking
  class Engine < ::Rails::Engine
    config.autoload_paths << File.expand_path("../..", __FILE__)
    config.autoload_paths << File.expand_path("../../../app/models/concerns", __FILE__)
    
    initializer 'configure ember-rails ranking', before: 'ember_rails.setup_vendor' do
      config.handlebars.templates_root << 'voluntary_ranking/templates'
    end
  end
end

--- 
+++ 
@@ -2,9 +2,13 @@
   class Engine < ::Rails::Engine
     config.autoload_paths << File.expand_path("../..", __FILE__)
     config.autoload_paths << File.expand_path("../../../app/models/concerns", __FILE__)
-    
+     
     initializer 'configure ember-rails ranking', before: 'ember_rails.setup_vendor' do
       config.handlebars.templates_root << 'voluntary_ranking/templates'
     end
+    
+    config.to_prepare do
+      User.send :include, Concerns::Model::User::Ranking
+    end
   end
 end
Include ranking module in user through config.to_prepare.
