model.sum(:col,:group=>'col1,col2') only return 2 columns?

There are 3 columns in my model 'Bank':bank_name,money,save_year,and I
want to get the sum money group by bank_name and save_year.
However,Bank.sum(:money,:group=>'bank_name,save_year') just returns the
sum money and the save_year column,and
Bank.sum(:money,:group=>'save_year,bank_name') just returns the sum
money and the bank_name column!How can I get all of the three
columns?And it's a rails feature or bug?My db is mysql,and the sql of
rails 2.1 generated returns all of the three columns in mysql query
Any help will be appreciated!