With this approach mongoid will fire only one count query

The previous version caused 3 query:
1. collection.empty? => load an item to ensure there are items in the collection
2. collection.first ... => again load an item to be sure about we querying mongo
3. collection.count(true) => actual count command

Author: trekdemo

lib/active_admin/mongoid/helpers/collection.rb

@@ -4,13 +4,15 @@ module Collection
 
       alias original_collection_size collection_size
       original_collection_size = instance_method(:collection_size)
+
       def collection_size(collection=collection)
-        if(not collection.empty? and collection.first.class.included_modules.include?(Mongoid::Document))
+        if collection.is_a?(::Mongoid::Criteria)
           collection.count(true)
         else
           original_collection_size(collection)
         end
       end
+
     end
   end
 end