The fundamental problem is that in order to increment the counter when
an existing record was associated, an extra query would be required to
determine whether that record was already included in the count. At
that point the performance gain of the counter is lost since you might
as well just query for the COUNT again. Then there's the whole issue
of deleting posts. I'm assuming the counter would not decrement
unless you call @forum.posts.delete(id).
In my experience the counter caches are scarily brittle. If the given
model is ONLY created through the association then they can be
manageable and offer a slight performance benefit, but if you have the
proper index, a COUNT(*) query is extremely fast.