Postgres Array存二维数组

https://dockyard.com/blog/ruby/2012/09/18/rails-4-sneak-peek-postgresql-array-support

二维数组必须每组数量一致， 可以缺值用nil代替

      # postgres array中只能存square array, 缺值用nil替代
      max_count = origin_ids.map{|item| item.size}.max
      origin_ids.map{|item| item + Array.new(max_count - item.size, nil)}