Unknown column problem in datamapper

[eluser]asmode[/eluser]
I am using datamapper.
I want to take count between related two tables, but i take an error like this:


Error Number: 1054
Unknown column 'table2.table2_id' in 'where clause'
SELECT COUNT(*) AS `numrows` FROM (`table2_table1`) WHERE `table2`.`table2_id` = '1' AND `table1_id` = '3'

I use examples in this page: http://stensi.com/datamapper/pages/count.html

what should i do?