Add resource_relation to Polymorphic concern

This allows suggest action to send search to
a relation instead of directly the model. Doing
so, the suggested items can be previously filtered.

app/controllers/concerns/commentable_actions.rb

@@ -34,7 +34,7 @@ module CommentableActions
 
   def suggest
     @limit = 5
-    @resources = @search_terms.present? ? resource_model.search(@search_terms) : nil
+    @resources = @search_terms.present? ? resource_relation.search(@search_terms) : nil
   end
 
   def create

app/controllers/concerns/polymorphic.rb

@@ -10,6 +10,10 @@ module Polymorphic
       @resource_name ||= resource_model.to_s.downcase
     end
 
+    def resource_relation
+      @resource_relation ||= resource_model.all
+    end
+
     def set_resource_instance
       instance_variable_set("@#{resource_name}", @resource)
     end