gistfile1.rb

# /config/application.rb
config.middleware.insert_before ActionDispatch::ParamsParser, "SelectiveStack"

# /config/initializers/omniauth.rb
::OmniAuthConfig = Proc.new do
  provider :github, # ...
end

# /app/middleware
class SelectiveStack
  def initialize(app)
    @app = app
  end

  def call(env)
    if env["PATH_INFO"].start_with?("/api/")
      @app.call(env)
    else
      middleware_stack.build(@app).call(env)
    end
  end

private
  def middleware_stack
    @middleware_stack ||= begin
      ActionDispatch::MiddlewareStack.new.tap do |middleware|
        # needed for OmniAuth
        middleware.use ActionDispatch::Cookies
        middleware.use Rails.application.config.session_store, Rails.application.config.session_options
        middleware.use OmniAuth::Builder, &OmniAuthConfig
        # needed for Doorkeeper /oauth views
        middleware.use ActionDispatch::Flash
      end
    end
  end
end