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