I want to filter in subquery with aliased field from main query:

select a.created_at::date as creation_date, a.field as field, 
       (select count(*)
        from table b
        where a.field = b.field
          and a.created_at::date = b.created_at::date) as count_one 
from table a
group by creation_date, field;

but this gives error
ERROR: subquery uses ungrouped column "a.created_at" from outer query

Any options how to solve it?