Rails SQL查询（如果为null）

我正在尝试使用lambda函数通过传入一个值来实现rails作用域：

"scope :for_rank, lambda {|rank| where('min_rank <= ? AND max_rank >= ?', rank, rank)}"

我希望搜索排名（15）时应同时返回a和b。请尝试以下操作：

 scope :for_rank, lambda {|rank| where('(min_rank <= ? AND max_rank IS NOT NULL AND max_rank >= ?) OR (min_rank <= ? AND max_rank IS NULL)', rank, rank, rank)}

scope:for_-rank，lambda{| rank |其中（'（min_-rank=？）或（min_-rank
 scope :for_rank, lambda {|rank| where('(min_rank <= ? AND max_rank IS NOT NULL AND max_rank >= ?) OR (min_rank <= ? AND max_rank IS NULL)', rank, rank, rank)}