capistrano_once.rb

module Capistrano
  class Configuration
    module Servers
      # Identifies all servers that the given task should be executed on.
      # The options hash accepts the same arguments as #find_servers, and any
      # preexisting options there will take precedence over the options in
      # the task.
      def find_servers_for_task(task, options={})
        find_servers(task.options.merge(options))
      end

      # Attempts to find all defined servers that match the given criteria.
      # The options hash may include a :hosts option (which should specify
      # an array of host names or ServerDefinition instances), a :roles
      # option (specifying an array of roles), an :only option (specifying
      # a hash of key/value pairs that any matching server must match), and
      # an :exception option (like :only, but the inverse).
      #
      # Additionally, if the HOSTS environment variable is set, it will take
      # precedence over any other options. Similarly, the ROLES environment
      # variable will take precedence over other options. If both HOSTS and
      # ROLES are given, HOSTS wins.
      #
      # Yet additionally, if the HOSTFILTER environment variable is set, it
      # will limit the result to hosts found in that (comma-separated) list.
      #
      # Usage:
      #
      #   # return all known servers
      #   servers = find_servers
      #
      #   # find all servers in the app role that are not exempted from
      #   # deployment
      #   servers = find_servers :roles => :app,
      #                :except => { :no_release => true }
      #
      #   # returns the given hosts, translated to ServerDefinition objects
      #   servers = find_servers :hosts => "[email protected]"
      def find_servers(options={})
        hosts  = server_list_from(ENV['HOSTS'] || options[:hosts])
        
        if hosts.any?
          filter_server_list(hosts.uniq)
        else
          roles  = role_list_from(ENV['ROLES'] || options[:roles] || self.roles.keys)
          only   = options[:only] || {}
          except = options[:except] || {}
          
          servers = roles.inject([]) { |list, role| list.concat(self.roles[role]) }
          servers = servers.select { |server| only.all? { |key,value| server.options[key] == value } }
          servers = servers.reject { |server| except.any? { |key,value| server.options[key] == value } }
          
          #allows you to add the option :once to a task ie: task :my_task, :roles => :app, :once => true do ...
          servers = [servers.first] if options[:once]
          logger.trace "servers: #{servers.map { |s| s.host }.inspect}"
          
          filter_server_list(servers.uniq)
        end
      end
    end
  end
end

namespace :deploy do

  desc <<-DESC
    Run the migrate rake task. By default, it runs this in most recently \
    deployed version of the app. However, you can specify a different release \
    via the migrate_target variable, which must be one of :latest (for the \
    default behavior), or :current (for the release indicated by the \
    `current' symlink). Strings will work for those values instead of symbols, \
    too. You can also specify additional environment variables to pass to rake \
    via the migrate_env variable. Finally, you can specify the full path to the \
    rake executable by setting the rake variable. The defaults are:

      set :rake,           "rake"
      set :rails_env,      "production"
      set :migrate_env,    ""
      set :migrate_target, :latest
  DESC
  task :migrate, :roles => :app, :once => true do
    rake = fetch(:rake, "rake")
    rails_env = fetch(:rails_env, "production")
    migrate_env = fetch(:migrate_env, "")
    migrate_target = fetch(:migrate_target, :latest)

    directory = case migrate_target.to_sym
      when :current then current_path
      when :latest  then current_release
      else raise ArgumentError, "unknown migration target #{migrate_target.inspect}"
      end

    run "cd #{directory}; #{rake} RAILS_ENV=#{rails_env} #{migrate_env} db:migrate"
  end
end