Monkey patch for OmniAuth::Strategy to make it Rack::Config configurable. Here we also include sample middleware that sets different configuration within an app serving many domains.

configuration-middleware.rb

module App
  module Middleware
    class OmniAuthConfigurator
      def initialize(app)
        @app = app
      end

      def call(env)
        current_domain = env["SERVER_NAME"] || env["HTTP_X_FORWARDED_FOR"]
        website = Website.find_by_domain(current_domain)

        if website.present?
          env["omniauth.facebook.client_id"] = website.facebook_app_id
          env["omniauth.facebook.client_secret"] = website.facebook_app_secret
        end

        @app.call(env)
      end
    end
  end
end

omniauth-strategy.rb

module OmniAuth

  module Strategy

    def call!(env)
      @env = env

      ### The custom part that looks for Rack::Config-uration
      #
      client_id = @env["omniauth.#{self.name}.client_id"]
      client_secret = @env["omniauth.#{self.name}.client_secret"]

      if client_id
        self.client.id = client_id
        self.client.secret = client_secret
      end
      ###

      if request.path == "#{OmniAuth.config.path_prefix}/#{name}"
        request_phase
      elsif request.path == "#{OmniAuth.config.path_prefix}/#{name}/callback"
        callback_phase
      else
        if respond_to?(:other_phase)
          other_phase
        else
          call_app!
        end
      end
    end
  end
end