build_errors - flatten

private def build_errors(contract:, error_prefix: nil)
  result = {}

  contract.attributes.each do |attribute_name, attribute|
    key = "#{error_prefix}#{attribute_name}"
    errors = contract.errors.full_messages_for(attribute_name)
    result[key] = errors if errors.present?

    if attribute.kind_of?(Array)
      attribute.each.with_index do |child_contract, i|
        child_errors = build_errors(contract: child_contract, error_prefix: "#{key}[#{i}].")

        result.merge!(child_errors)
      end
    end
  end

  (contract.errors.keys.map(&:to_s) - result.keys - contract.attributes.keys).each do |key|
    errors = contract.errors.full_messages_for(key)
    result[key] = errors if errors.present?
  end

  result
end