rails/rails — comment on commit fc956425d405e75bcf310f3e6f08cf201cc8131a
File: actionpack/test/template/form_helper_test.rb line 1020
By josevalim:

The documentation for `to_key` is very clear: "returns a key regardless if the object is persisted or not". If someone was relying on the old behavior, they were wrong :). In any case, this is a Rails 4 change, so it gives us plenty of mileage to detect issues.