redis 是非常强大的key-value数据库，性能特别高，适合大规模数据处理。不过，据说其数据存在在内存中，所以，还是尽量的少放些数据到其中吧。最好只放一个id号。

1. 下载安装redis
   http://redis.io/download
   安装命令，大同小异
   

   curl -O http://redis.googlecode.com/files/redis-2.2.2.tar.gz
   tar xzf redis-2.2.2.tar.gz
   cd redis-2.2.2
   make
   cp src/redis-server src/redis-cli /usr/bin

2. 启动redis服务器
   

   redis-server

   或者
   

   redis-server   /path/to/redis.conf

   redis.conf的配置文件参看地址  安装目录下可以找到
   
3. redis for rails
   https://github.com/redis/redis-rb
   

4. gem installgem 'redis'  #rails3
   config.gem 'redis'    #rails2 

   
   
5. connect redis
   创建 redis.rb
   

   $redis = Redis.new(:host => 'localhost', :port => 6379)

   
   
6. 如下方法来自参考的链接
   

   class User < ActiveRecord::Base# follow a userdef follow!(user)$redis.multi do$redis.sadd(self.redis_key(:following), user.id)$redis.sadd(user.redis_key(:followers), self.id)endend# unfollow a userdef unfollow!(user)$redis.multi do$redis.srem(self.redis_key(:following), user.id)$redis.srem(user.redis_key(:followers), self.id)endend# users that self followsdef followersuser_ids = $redis.smembers(self.redis_key(:followers))User.where(:id => user_ids)end# users that follow selfdef followinguser_ids = $redis.smembers(self.redis_key(:following))User.where(:id => user_ids)end# users who follow and are being followed by selfdef friendsuser_ids = $redis.sinter(self.redis_key(:following), self.redis_key(:followers))User.where(:id => user_ids)end# does the user follow selfdef followed_by?(user)$redis.sismember(self.redis_key(:followers), user.id)end# does self follow userdef following?(user)$redis.sismember(self.redis_key(:following), user.id)end# number of followersdef followers_count$redis.scard(self.redis_key(:followers))end# number of users being followeddef following_count$redis.scard(self.redis_key(:following))end# helper method to generate redis keysdef redis_key(str)"user:#{self.id}:#{str}"end
   end
   

   
   follow, following 关系体现在redis 中
7. redis还能用于 缓存
   https://github.com/jodosha/redis-store
   
   
   
   
   
   
   
   
   
   
   

参考：
http://redis.io/

http://jimneath.org/2011/03/24/using-redis-with-ruby-on-rails.html
https://github.com/redis/redis-rb