I had the same issue, the possible solution you suggest does not work because the session is not shared between subdomains.
I solved it the following way (same idea you propossed, different implementation):
- Create a new model (I called it LoginKey) that contains the user_id and a random SHA1 key.
- When the user is authenticated at the parent domain (for example: mydomain.com/users/sign_in), a new LoginKey is created and the user is redirected to the corresponding subdomain to an action that I called login_with_key (for example: user_subdomain.mydomain.com/users/login_with_key?key=f6bb001ca50709efb22ba9b897d928086cb5d755322a3278f69be4d4daf54bbb)
Automatically log the user in with the key provided:
key = LoginKey.find_by_login_key(params[:key])
sign_in(key.user) unless key.nil?
Destroy the key:
key.destroy
I didn't like this solution 100%, I tried out a lot of different approaches that do not require a db record to be created, but always faced security concerns, and I think this one is safe.