an example for syncing between mongodb and mysql databases

sync.rb

require 'mongo'
require 'mysql'

include Mongo

mongo_connection = MongoClient.new("localhost", 27017)

itunes_db = mongo_connection.db("itunes")

@collection = itunes_db["itunes"]

@itunes_array = @collection.find("type" => "track").to_a

@itunes_array.each do |record|
  begin
    mysql_db = Mysql.new('localhost', 'root', 'root', 'itunes')

    insert_track = mysql_db.prepare "insert into itunes (persistentid,album,artist,size,kind,year,playcount,lastmodified) values('#{record['PersistentID'].to_s.gsub("'","\\\\'")}','#{record['Album'].to_s.gsub("'","\\\\'")}','#{record['Artist'].to_s.gsub("'","\\\\'")}','#{record['Size']}','#{record['Kind'].to_s.gsub("'","\\\\'")}','#{record['Year']}','#{record['TrackCount']}','#{record['DateModified']}') on duplicate key update album=album, artist=artist, size=size, year=year, playcount=playcount, lastmodified=lastmodified;"
    insert_track.execute
  ensure
    mysql_db.close
  end
end

begin
  mysql_db = Mysql.new('localhost', 'root', 'root', 'itunes')
  all_itunes = mysql_db.query("SELECT * FROM itunes")
  all_itunes.each_hash do |row|
    @collection.save({PersistentID: "#{row['persistentid']}", Album: "#{row['album']}", Artist: "#{row['artist']}", Size: "#{row['size']}", Kind: "#{row['kind']}", Year: "#{row['year']}", TrackCount: "#{row['playcount']}", DateModified: "#{row['lastmodified']}"}) unless @itunes_array.any? { |record| record['PersistentID'] == row['persistentid'] }
  end
ensure
  mysql_db.close
end

Hi it updates all rows not delta updates so it is not really sync.