Q:

`redirect_uri` is not under the registered domain for this application

I am trying to implement Stack Exchange authorization in my RoR application. 
But I get the error :

redirect_uri is not under the registered domain for this application

in Gemfile use: 
gem 'devise'
gem 'omniauth'
gem 'omniauth-stackexchange'

in devise.rb: 
 config.omniauth: stackexchange, 'CLIENT_ID', 'CLIENT SECRET', callback_url: 'stackexchange.com'

Any ideas?

A:

I don't know about that Ruby/Gem, but your app registration is not correct.  
You need to configure your app for explicit OAuth2, per this other answer, except enter localhost instead of example.com.
(The localhost is based on the original version of the question.  If you have a registered domain, use that.)
Then, in your ruby code:
config.omniauth: stackexchange, 'CLIENT_ID', 'CLIENT SECRET', callback_url: 'http://localhost:3000/'

(Assuming the rest of that devise.rb code was correct.  But see the OmniAuth StackExchange doc/GitHub about that.)

