discourse/lib/validators/topic_title_length_validator.rb

class TopicTitleLengthValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, value)
    title_validator(record).validate_each(record, attribute, value)
  end

  private

    def title_validator(record)
      length_range =
        if record.user.try(:admin?)
          1..SiteSetting.max_topic_title_length
        elsif record.private_message?
          SiteSetting.private_message_title_length
        else
          SiteSetting.topic_title_length
        end

      ActiveModel::Validations::LengthValidator.new(attributes: :title, in: length_range, allow_blank: true)
    end

end