In the Rails 3 docs, the build method for associations is described as being the same as the new method, but with the automatic assignment of the foreign key. Straight from the docs: Firm#clients.build (similar to Client.new("firm_id" => id)) I've read similar elsewhere. However, when I use new (e.g. some_firm.clients.new without any parameters), the new client's firm_id association is automatical