# db/migrate/*_create_account_identities.rb
class CreateAccountIdentities < ActiveRecord::Migration
  def change
    create_table :account_identities do |t|
      t.references :account, foreign_key: { on_delete: :cascade }
      t.string :provider, null: false
      t.string :uid, null: false
      t.index [:provider, :uid], unique: true
    end
  end
end

# app/misc/rodauth_main.rb
class RodauthMain < Rodauth::Rails::Auth
  configure do
    enable :omniauth
    omniauth_provider :facebook,
      Rails.application.credentials.facebook[:app_id],
      Rails.application.credentials.facebook[:app_secret],
      scope: "email"
  end
end