ActiveRecord find :select

Hello,

Why using this :select, it works :

:select=>"*, (select count(*) from `tasks` T) as subcache"

but using this one does not:

:select=>"*, (select count(*) from `tasks` T where T.task_id =
`tasks`.id) as subcache"

returning this error:

ActionView::TemplateError (undefined method `subcache' for
#<Task:0xb706bc30>) on line #6 of task/_task_c.rhtml:
6: <% nb = task_c.subcache.to_i %>

I have tested this query in MySQL Query Broser and it is well formed and
works.
select *, (select count(*) from tasks T where T.task_id = tasks.id) as
subcache from tasks

Thanks,
Mickael.