Convert Redmine wiki and issues from pandoc to markdown, based on http://www.hitmaroc.net/216445-2249-how-convert-existing-redmine-wiki-textile-markdown.html

convert_textile_to_markdown.rb

task :convert_textile_to_markdown => :environment do
  require 'tempfile'
  WikiContent.all.each do |wiki|
    ([wiki] + wiki.versions).each do |version|
      textile = version.text
      markdown = convert_to_markdown textile
      puts "-- Changing the text of wiki ##{wiki.id} to:"
      puts markdown
      version.update_attribute(:text, markdown)
    end
  end
  Issue.all.each do |issue|
    textile = issue.description
    unless textile.nil?
      markdown = convert_to_markdown textile
      issue.update_attribute(:description, markdown)
      puts "-- Changing the text of issue ##{issue.id} to:"
      puts markdown
    end
    issue.journals.each do |journal|
      textile = issue.notes
      next if textile.nil?
      markdown = convert_to_markdown textile
      puts "--    Changing journal #{journal.id} for ##{issue.id} to:"
      puts "--    #{markdown}"
      journal.update_attribute(:notes, markdown)
    end
  end
end

def convert_to_markdown(textile)
  src = Tempfile.new('textile')
  src.write(textile)
  src.close
  dst = Tempfile.new('markdown')
  dst.close

  command = [
    "pandoc",
    "--no-wrap",
    "--smart",
    "-f",
    "textile",
    "-t",
    "markdown_strict",
    src.path,
    "-o",
    dst.path,
  ]
  system(*command) or raise "pandoc failed"

  dst.open
  markdown = dst.read

  # remove the \ pandoc puts before * and > at begining of lines
  markdown.gsub!(/^((\\[*>])+)/) { $1.gsub("\\", "") }

  # add a blank line before lists
  markdown.gsub!(/^([^*].*)\n\*/, "\\1\n\n*")
  markdown
end