authentication.rb

  def self.from_omniauth(auth,admins)
      where(auth.slice(:provider, :uid)).first_or_initialize.tap do |authentication|
          authentication.admin_id = admins.id
          authentication.provider = auth.provider
          authentication.uid = auth.uid
          authentication.oauth_token = auth.credentials.token
          authentication.oauth_expires_at = Time.at(auth.credentials.expires_at)
          authentication.save!
      end
  end

home_controller.rb

  def create
      admins = Admin.find(current_admin.id)
      user = Authentication.from_omniauth(env["omniauth.auth"], admins)
      redirect_to root_url
  end

works now