mongo task for rails ( put this task file as lib/tasks/mongo.rake )

mongo.rake

# -*- mode: ruby -*-

include FileUtils::Verbose

@root = File.expand_path( File.dirname(__FILE__) + '/../../' )

namespace :mongo do
  desc 'prepare'
  task :prepare do
    dbpath = "#{@root}/db/mongodb"
    mkdir( dbpath ) unless File.exist? dbpath

    conf = File.join( @root, 'config', 'mongo_develop.conf.erb' )
    if ( not File.exist? conf )
      open( conf, 'w' ) { |f|
        f.puts <<'EOD'
<% root = File.expand_path( File.dirname(__FILE__) + '/../' ) %>
dbpath  = <%= "#{root}/db/mongodb" %>
logpath = <%= "#{root}/log/mongodb.log" %>
EOD
      }
      puts "created #{conf} ."
    end
  end

  desc 'start mongod'
  task :start do
    conf = File.join( @root, 'config', 'mongo_develop.conf' )
    sh <<EOD
bundle exec erubis #{conf}.erb > #{conf}
mongod -f #{conf}
EOD
  end
end

sample mongo_develop.conf.erb

<% root = File.expand_path( File.dirname(__FILE__) + '/../' ) %>
dbpath  = <%= "#{root}/db/mongodb" %>
logpath = <%= "#{root}/log/mongodb.log" %>