sql – Rails：使用AREL进行批量属性查询

我想使用像find_in_batches这样的东西,但是我不想对完全实例化的AR对象进行分组,而是将某个属性组合起来,比如说,就像id一样.所以,基本上,使用find_in_batches和pluck的混合：

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(也许与Arel),而不必自己编写OFFSET / LIMIT逻辑或重复分页宝石如paginate或kaminari？

解决方法

这不是理想的解决方案,但是这里的方法只是复制粘贴大多数find_in_batches但产生关系而不是记录数组(未经测试) – 只需将其修补为关系：

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

这不是最好的实现(特别是关于实例化记录的primary_key_offset计算),但是你得到了精神.