sql - rails : batched attribute queries using AREL

Question

我想使用类似 find_in_batches 的东西，但不是对完全实例化的 AR 对象进行分组，我想对某个属性进行分组，例如 id。所以，基本上，混合使用 find_in_batches和 pluck :

Cars.where(:engine => "Turbo").pluck(:id).find_in_batches do |ids|
  puts ids
end

# [1, 2, 3....]
# ...

有没有办法做到这一点(也许用 Arel)，而不必自己编写 OFFSET/LIMIT 逻辑或重复分页 gems 像 will paginate 或 kaminari？

Answer 1

这不是理想的解决方案，但这里有一种方法只是复制粘贴 find_in_batches 的大部分内容。但产生一个关系而不是一组记录(未经测试) - 只需将其修补成 Relation :

def 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)
  batch_size = options.delete(:batch_size) || 1000

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

  while ( size = relation.size ) > 0    

    yield relation

    break if size < batch_size

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

有了这个，你应该能够做到:

Cars.where(:engine => "Turbo").in_batches do |relation|
  relation.pluck(:id)
end

这可能不是最好的实现(特别是关于primary_key_offset 计算，它实例化一个记录)，但你明白了。