Rails find_each/find_in_batches 只能设定 start,不能设置 end

ibachue · 2013年03月22日 · 最后由 bindiry 回复于 2016年06月07日 · 2714 次阅读

Hi all,

最近尝试把find_each方法并行化,因为经常要在数据库里修改错误数据,传统的方式实在效率不高,希望能够并行加快速度。 现在遇到一个问题就是,如果同时有几个find_each在跑的话,那必须设法规定每个find_each只负责多少范围的数据,这就使得必须有一个可以设置范围的参数。 但是从find_each/find_in_batches的实现当中,却只能找到start参数,却没有一个与之相对应的end参数,(虽然也可以用batch_size参数,但是这个参数代表另一个意义,不能乱用)感觉非常奇怪。为什么Rails不提供这样的功能呢?难道设置一个end参数会导致什么问题嘛? 谢谢

这里贴一下2.3的实现:

module ActiveRecord
  module Batches # :nodoc:
    def self.included(base)
      base.extend(ClassMethods)
    end

    module ClassMethods
      def find_each(options = {})
        find_in_batches(options) do |records|
          records.each { |record| yield record }
        end

        self
      end

      def find_in_batches(options = {})
        raise "You can't specify an order, it's forced to be #{batch_order}" if options[:order]
        raise "You can't specify a limit, it's forced to be the batch_size"  if options[:limit]

        start = options.delete(:start).to_i
        batch_size = options.delete(:batch_size) || 1000

        proxy = scoped(options.merge(:order => batch_order, :limit => batch_size))
        records = proxy.find(:all, :conditions => [ "#{table_name}.#{primary_key} >= ?", start ])

        while records.any?
          yield records

          break if records.size < batch_size

          last_value = records.last.id

          raise "You must include the primary key if you define a select" unless last_value.present?

          records = proxy.find(:all, :conditions => [ "#{table_name}.#{primary_key} > ?", last_value ])
        end
      end


      private
        def batch_order
          "#{table_name}.#{primary_key} ASC"
        end
    end
  end
end

3.2实现也没有什么进步:

require 'active_support/core_ext/object/blank'

module ActiveRecord
  module Batches
    def find_each(options = {})
      find_in_batches(options) do |records|
        records.each { |record| yield record }
      end
    end

    def find_in_batches(options = {})
      relation = self

      unless arel.orders.blank? && arel.taken.blank?
        ActiveRecord::Base.logger.warn("Scoped order and limit are ignored, it's forced to be batch order and batch size")
      end

      if (finder_options = options.except(:start, :batch_size)).present?
        raise "You can't specify an order, it's forced to be #{batch_order}" if options[:order].present?
        raise "You can't specify a limit, it's forced to be the batch_size"  if options[:limit].present?

        relation = apply_finder_options(finder_options)
      end

      start = options.delete(:start).to_i
      batch_size = options.delete(:batch_size) || 1000

      relation = relation.reorder(batch_order).limit(batch_size)
      records = relation.where(table[primary_key].gteq(start)).all

      while records.any?
        records_size = records.size
        primary_key_offset = records.last.id

        yield records

        break if records_size < batch_size

        if primary_key_offset
          records = relation.where(table[primary_key].gt(primary_key_offset)).to_a
        else
          raise "Primary key not included in the custom select clause"
        end
      end
    end

    private

    def batch_order
      "#{quoted_table_name}.#{quoted_primary_key} ASC"
    end
  end
end
共收到 5 条回复
  1. 不明白 batch_size 代表什么特殊意义

  2. 一般 sql 都是

    select * from users offset 100 limit 100 
    

    没用过

    select from users offset 100 end 200...
    

#1楼 @zgm batch_size就是一次拽下多少数据 相当于limit 为了防止数据过多导致服务器撑爆

#2楼 @iBachue 我知道他是 limit,但是不知道危险在哪, 默认是1000 挺好的。

sql 里面就没有 end 这种 语法, 也只有 offset 这种语法, 如果实在要用可以加 where(id < xxx), 但这明显也不是你要的。

http://guides.rubyonrails.org/active_record_querying.html

http://api.rubyonrails.org/classes/ActiveRecord/Batches.html#method-i-find_each

By default, records are fetched in ascending order of the primary key, which must be an integer. The :start option allows you to configure the first ID of the sequence whenever the lowest ID is not the one you need. This would be useful, for example, if you wanted to resume an interrupted batch process, provided you saved the last processed ID as a checkpoint.



Another example would be if you wanted multiple workers handling the same processing queue. You could have each worker handle 10000 records by setting the appropriate :start option on each worker.
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