什么时候使用"|| =" Rails中的运算符?它的意义是什么? [英] When do we use the "||=" operator in Rails ? What is its significance?

问题描述

可能重复:
操作符|| =在红宝石中代​​表什么?

Possible Duplicate:
What does the operator ||= stands for in ruby?

我对Rails中||=运算符的用法感到困惑.我无法在网络上找到任何有用的信息.谁能指导我?

I am confused with the usage of ||= operator in Rails. I couldn't locate anything useful on the web. Can anyone please guide me?

请让我知道您是否知道任何网络链接.

Do let me know if there are any weblinks that you are aware of.

我想要以下声明的意思:

I would like what the following statement means:

@_current_user ||= session[:current_user_id] &&
      User.find(session[:current_user_id])

推荐答案

让我们分解一下:

@_current_user ||= {SOMETHING}

这就是说，如果@_current_user为nil，false或未定义，则将其设置为{SOMETHING}.否则，将其设置为@_current_user，或者什么也不做.扩展形式:

This is saying, set @_current_user to {SOMETHING} if it is nil, false, or undefined. Otherwise set it to @_current_user, or in other words, do nothing. An expanded form:

@_current_user || @_current_user = {SOMETHING}

好，现在到右侧.

session[:current_user_id] &&
      User.find(session[:current_user_id])

您通常会看到&&仅带有布尔值，但是在Ruby中，您不必这样做.这里的技巧是，如果session[:current_user_id]不是nil，并且User.find(session[:current_user_id])不是nil，则表达式的计算结果为User.find(session[:current_user_id])，否则为nil.

You usually see && with boolean only values, however in Ruby you don't have to do that. The trick here is that if session[:current_user_id] is not nil, and User.find(session[:current_user_id]) is not nil, the expression will evaluate to User.find(session[:current_user_id]) otherwise nil.

所以用伪代码将它们放在一起:

So putting it all together in pseudo code:

if defined? @_current_user && @_current_user
  @_current_user = @_current_user
else
  if session[:current_user_id] && User.find(session[:current_user_id])
    @_current_user = User.find(session[:current_user_id])
  else
    @_current_user = nil
  end
end

这篇关于什么时候使用"|| =" Rails中的运算符?它的意义是什么?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！