Setting Devise omniauth_path_prefix doesn't work - ruby-on-rails-3

Question

I'm working on a Rails-based API. I recently started attempting to version it. (I'm using the Versionist gem, in case it matters) One version ('v2') uses Devise and Omniauth to authenticate users through Facebook/Twitter.
I want all the routes associated with this version to have the appropriate version prefix (so users/:username/foo becomes v2/users/:username/foo, etc.), but I've already found out that putting devise_for inside the api_version block prevents the Devise helpers (current_user, user_signed_in?, etc.) from working, so it continues to live outside the block:
routes.rb:
devise_for :user, :path => '', :controllers => {:omniauth_callbacks => 'users/omniauth_callbacks'}, :skip => [:registrations, :confirmations, :sessions, :passwords]
api_version(:module => "V2", :path=>"v2") do
resources :authentications, :only => [:update, :destroy]
devise_scope :user do
post 'login' => 'sessions#create', :as => 'user_session'
get 'logout' => 'sessions#destroy'
post 'password' => 'devise/passwords#create'
put 'password' => 'devise/passwords#update'
end
end
Everything seemed great... except the Devise-generated omniauth routes:
rake routes output:
user_omniauth_authorize /auth/:provider(.:format)
user_omniauth_callback /auth/:action/callback(.:format)
Now, some google-fu revealed that there's a devise configuration setting for this, so I added the following to our devise initializer (config/initializers/devise.rb):
Devise.setup do |config|
config.omniauth_path_prefix = 'v2/auth'
end
Now, rake routes produces paths that look sensible:
user_omniauth_authorize /v2/auth/:provider(.:format) v2/users/omniauth_callbacks#passthru {:provider=>/(?!)/}
user_omniauth_callback /v2/auth/:action/callback(.:format) v2/users/omniauth_callbacks#(?-mix:(?!))
However, when I attempt to access this route by calling api.localhost/v2/auth/facebook, I get a routing error:
ActionController::RoutingError (No route matches [GET] "/v2/auth/facebook")
Any idea what's going on here?

Answer 1

You are missing the provider name in the routes so they don't match the facebook part in /v2/auth/facebook. The correct route destination should look something like v2/users/omniauth_callbacks#(?-mix:facebook).
Have you specified the provider in the user model?
devise_for ..., :omniauthable, :omniauth_providers => [:facebook]
For the record, I'm using Rails 3.2 and Devise 3.0 and the altered route seems to work (I haven't gone further yet to see if something else will break).