Before you're able to use Doorkeeper, you need to configure how resource owners (users) can be authenticated and who can manage such applications.

Resource Owner Authentication

This configuration should do two things:

1. Return the user is currently authenticated

2. Redirect the user to the authentication page

If you're using devise, one option is to write the following:

config/initializers/doorkeeper.rb
Doorkeeper.configure do
  resource_owner_authenticator do
    current_user || warden.authenticate!(scope: :user)
  end
end

The block above runs in the context of your application so you have access to your models, session and routes helpers. However, it is not run in the context of the ApplicationController which means that it doesn't have access to the methods defined over there.

You may want to check other ways of authentication here.

Application Management Authentication

By default, the applications list in /oauth/applications is unavailable. To let users see and manage all applications, you should configure admin_authenticator block:

config/initializers/doorkeeper.rb
# config/initializers/doorkeeper.rb
Doorkeeper.configure do
  admin_authenticator do |routes|
    current_user || warden.authenticate!(scope: :user)
  end
end

The block follows the same rules as resource_owner_authenticator block.