Rails:状态与状态的比较失败

Question

我需要获取所有current_user.friends状态,然后按created_at对它们进行排序.

class User < ActiveRecord::Base
 has_many :statuses
end

class Status < ActiveRecord::Base
 belongs_to :user
end

在控制器中:

def index
    @statuses = []
    current_user.friends.map{ |friend| friend.statuses.each { |status| @statuses << status } }
    current_user.statuses.each { |status| @statuses << status }

    @statuses.sort! { |a,b| b.created_at <=> a.created_at }
end

current_user.friends 返回一个对象数组 User

friend.statuses 返回一个对象数组 Status

错误:

comparison of Status with Status failed
app/controllers/welcome_controller.rb:10:in `sort!'
app/controllers/welcome_controller.rb:10:in `index'

Answer 1

我有一个类似的问题,用to_i方法解决,但无法解释为什么会发生这种情况.

@statuses.sort! { |a,b| b.created_at.to_i <=> a.created_at.to_i }

顺便说一句,这按降序排序.如果你想升序是:

@statuses.sort! { |a,b| a.created_at.to_i <=> b.created_at.to_i }

Answer 2

当sort从<=>返回nil时,会出现此错误消息.<=>可以返回-1,0,1或nil,但sort不能处理nil,因为它需要所有列表元素具有可比性.

class A
  def <=>(other)
    nil
  end
end

[A.new, A.new].sort
#in `sort': comparison of A with A failed (ArgumentError)
#   from in `<main>'

调试此类错误的一种方法是检查<=>的返回值是否为nil,如果是则返回异常.

@statuses.sort! do |a,b| 
  sort_ordering = b.created_at <=> a.created_at
  raise "a:#{a} b:#{b}" if sort_ordering.nil?
  sort_ordering
end