validates presence of either email or username?

email_username.rb

  validate :has_unique_email_or_username, on: :create

  def has_unique_email_or_username
    if username.nil? && email.nil?
      errors.add(:email, "valid email or username required")
    elsif not email.match(/\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/)
      errors.add(:email, "please enter a valid email")
    elsif User.where(email: email).count > 0
      errors.add(:email, "that email has already been taken")
    elsif User.where(username: username).count > 0
      errors.add(:username, "that username has already been taken")
    end
  end