You can use the model.update_counters method to update the counter_cache column. But if you have a million rows it be very fast.
So for large tables it's best to do it with a query such as this:
update categories, (select id as category_id, ifnull(count, 0) as count from categories left join (select category_id, count(id) as count from products group by category_id) as count_table on categories.id = count_table.category_id) as count_table set categories.products_count = count_table.count where categories.id = count_table.category_id
This query updates the count for all rows.
The code needs to be modified for your database design.