capistrano db backup task from http://www.railsfever.com/blogs/automatic-database-backups-with-capistrano-3

db_backup.cap

desc "Backup the database"
namespace :db do
    task :backup do
        on roles(:db) do |host|
            backup_path = "#{fetch(:deploy_to)}/backups"
            execute :mkdir, "-p #{backup_path}"
            basename = 'database'

            username, password, database, host = get_remote_database_config(fetch(:stage))
            debug "#{username}, #{password}, #{database}"

            filename = "#{basename}_#{fetch(:stage)}_#{database}_#{Time.now.strftime '%Y-%m-%d_%H:%M:%S'}.sql.bz2"
            debug "We will backup to file: #{backup_path}/#{filename}"

            hostcmd = host.nil? ? '' : "-h #{host}" 
            execute :mysqldump, "-u #{username} --password='#{password}' --databases #{database} #{hostcmd} 
                | bzip2 -9 > #{backup_path}/#{filename}"

            purge_old_backups "#{basename}", "#{backup_path}"
        end
    end

    def get_remote_database_config(db)      
        remote_config = capture("cat #{shared_path}/config/database.yml")
        database = YAML::load(remote_config)
        return database["#{db}"]['username'], database["#{db}"]['password'], database["#{db}"]['database'], 
            database["#{db}"]['host']    
    end

  def purge_old_backups(basename,backup_path)
    max_keep = fetch(:keep_db_backups, 5).to_i
    backup_files = capture("ls -t #{backup_path}/#{basename}*").split.reverse
    if max_keep >= backup_files.length
      info "No old database backups to clean up"
    else
      info "Keeping #{max_keep} of #{backup_files.length} database backups"
      delete_backups = (backup_files - backup_files.last(max_keep)).join(" ")
      execute :rm, "-rf #{delete_backups}"
    end
  end
end

deploy.rb

#settings
set :keep_db_backups, 10

#execution
namespace :deploy do
    before :updating, 'db:backup'

    #other code

end