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.