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