This article is the source of this code, but I’ve included some of the recommended modifications in the comments. This will back up a MySQL database (via mysqldump) into a directory of your choosing (or RAILS_ROOT/db by default), rolling automatically at 20 backups, and using your database.yml as the source of the login information (so you keep it DRY).

require 'find' 
  namespace :db do  desc "Backup the database to a file. Options: DIR=base_dir 
RAILS_ENV=production MAX=20"
    task :backup => [:environment] do
      datestamp = Time.now.strftime("%Y-%m-%d_%H-%M-%S")    
      base_path = ENV["DIR"] || "db"
      backup_base = File.join(base_path, 'backup')
      backup_folder = File.join(backup_base, datestamp)
      backup_file = File.join(backup_folder, "#{RAILS_ENV}_dump.sql.gz")    
      File.makedirs(backup_folder)    
      db_config = ActiveRecord::Base.configurations[RAILS_ENV]   
      sh "mysqldump -u #{db_config['username'].to_s} #{'-p' if db_config[
'password']}#{db_config['password'].to_s} --opt #{db_config['database']} | 
gzip -c > #{backup_file}"     
      dir = Dir.new(backup_base)
      all_backups = (dir.entries - ['.', '..']).sort.reverse
      puts "Created backup: #{backup_file}"     
      max_backups = ENV["MAX"] || 20
      unwanted_backups = all_backups[max_backups.to_i..-1] || []
      for unwanted_backup in unwanted_backups
      FileUtils.rm_rf(File.join(backup_base, unwanted_backup))
      puts "deleted #{unwanted_backup}"
    end
    puts "Deleted #{unwanted_backups.length} backups, #{all_backups.length - 
unwanted_backups.length} backups available"
  end
end

I created a cron entry like so to backup my mephisto database every day at 3am (EST):

0 3 * * * cd /srv/mephisto-0.8/ && /usr/local/bin/rake db:backup RAILS_ENV=production

Of course if your running on a VPS or hosted solution, your provider is likely doing their own backups, but if you’re not, this can come in handy.