How do you set the value of a newly added ActiveRecord counter cache?

Question

I have a model object which did not have a counter cache on it before and I added it via a migration. The thing is, I tried and failed to set the starting value of the counter cache based on the number of child objects I already had in the migration. Any attempt to update the cache value did not get written to the database. I even tried to do it from the console but it was never going to happen. Any attempt to write directly to that value on the parent was ignored.

Changing the number of children updated the counter cache (as it should), and removing the ":counter_cache => true" from the child would let me update the value on the parent. But that's cheating. I needed to be able to add the counter cache and then set its starting value to the number of children in the migration so I could then start with correct values for pages which would show it.

What's the correct way to do that so that ActiveRecord doesn't override me?

Answer 1

You want to use the update_counters method, this blog post has more details:

http://josh.the-owens.com/archives/2007/11/03/rails-edge-change-how-to-add-a-counter-cache-to-an-existing-db-table/

This RailsCasts on the topic is also a good resource:

http://railscasts.com/episodes/23-counter-cache-column