Remove group when building the relation, it will be added later if

options[:group] is given.

activerecord/lib/active_record/calculations.rb

@@ -149,7 +149,6 @@ def calculate(operation, column_name, options = {})
             arel_table(options[:from]).
               join(construct_join(options[:joins], scope)).
               where(construct_conditions(options[:conditions], scope)).
-              group(construct_group(options[:group], options[:having], scope)).
               order(options[:order]).
               take(options[:limit]).
               skip(options[:offset])