Cache SHOW COLUMNS

今天recity发布第二版本，优化了半天，终于达到一个稍微满意的速度，闲下来也写了篇bllog轻松下:)


这个东西据说在production已经cache了，是的，确实是的！

今天主要的目的是折腾 于是乎google一把，看有没有现成的plugin,呵呵，俺想偷懒

没找到plugins,但是找到一个不错的patch:http://dev.rubyonrails.org/attachment/ticket/9046/cache_column_names.patch

哦，原来就是这么个东西，开始动手，把它转换成plugins

先说明下，这个只是初步入门级别，没什么深入研究的可是为什么要写这篇文章：
呵呵，纯属折腾。。。

新建一个plugin

ruby script/generate plugin CacheColumns

将cache_column_name.patch的代码稍作修改填到我们的lib/cache_columns里去
这里面的columns和reset_column_information是实类方法，所以我们可以使用

base.instance_eval do
  block goes here
end

来覆盖原始的method

首先请先备份原始的两个method

        alias old_columns columns
        alias old_reset_column_information reset_column_information

新建一个全局的变量Hash来缓存columns

@@columns = {}

OK，include 到ActiveRecord::Base里去

ActiveRecord::Base.send :include, CacheColumns::ActiveRecord

打完收工

完整代码如下：
/vendor/plugins/cache_columns/init.rb

ActiveRecord::Base.send :include, CacheColumns::ActiveRecord

/vendor/plugins/cache_columns/lib/cache_columns.rb

module CacheColumns
  module ActiveRecord
    @@columns = {}

    def self.included(base)
      base.instance_eval do
        alias old_columns columns
        alias old_reset_column_information reset_column_information

        def columns
          if  @@columns[table_name].nil?
            @@columns[table_name] = connection.columns(table_name, "#{name} Columns")
            @@columns[table_name].each {|column| column.primary = column.name == primary_key}
          end
          @@columns[table_name]
        end

        #  # Resets all the cached information about columns, which will cause them to be reloaded on the next request.
        def reset_column_information
          generated_methods.each { |name| undef_method(name) }
          @column_names = @columns_hash = @content_columns = @dynamic_methods_hash = @read_methods = @inheritance_column = nil
          @@columns.delete(table_name)
        end

        def reset_column_cache #:nodoc:
          @@columns = {}
        end
      end
    end
end

代码我放到code.google.com上面去了

svn co http://cache-columns.googlecode.com/svn/trunk

哦，你想缓存到memcached中？好吧，我们再继续修改之,有过一次经历，这次再动手就简单多了。

require 'memcache_util'

module CacheColumns
  module ActiveRecord
    def self.included(base)
      base.instance_eval do
        alias old_columns columns
        alias old_reset_column_information reset_column_information
        @ttl = 60 * 30 #增加个配置选择?

        def columns
          record =  get_columns(table_name)
          unless record
            record = connection.columns(table_name, "#{name} Columns")
            record.each {|column| column.primary = column.name == primary_key}
            cache_store record
          end
          record
        end

        #  # Resets all the cached information about columns, which will cause them to be reloaded on the next request.
        def reset_column_information
          generated_methods.each { |name| undef_method(name) }
          @column_names = @columns_hash = @content_columns = @dynamic_methods_hash = @read_methods = @inheritance_column = nil
          cache_delete
        end

        #get columns from memcached
        def get_columns(name)
          start_time = Time.now
          record = Cache.get cache_key_memcache
          elapsed = Time.now - start_time
          ActiveRecord::Base.logger.debug('CacheColumns Get (%0.6f)  %s' % [elapsed, cache_key_memcache])
          record
        end

        #store columns
        def cache_store(record)
          start_time = Time.now
          Cache.put cache_key_memcache, record, @ttl
          elapsed = Time.now - start_time
          ActiveRecord::Base.logger.debug('CacheColumns Set (%0.6f)  %s' % [elapsed, cache_key_memcache])
          record
        end

        def cache_key_memcache
          "active_record:columns:#{table_name}"
        end

        def cache_delete
          Cache.delete  cache_key_memcache
        end
      end
    end
  end
end