新手问题 为何 rails c 里面下面这个会返回 false
首先有个 User model
class User < ApplicationRecord
end
然后作比较的时候会发现 某条记录的 class 居然不是 User
User.first.class === User
会返回 false
IChou
#1
2018年07月27日
User === User.first
# => true
User === User
# => false
Ruby 的 === 不是恒等，而是验证归属性的
=== has absolutely nothing whatsoever to do with equality. In particular, it violates pretty much every law that you would ecpect an equality operator to follow. And it does that very much by design. – Jörg W Mittag Dec 17 '10 at 5:08 [1]
(1..5) === 3 # => true
(1..5) === 6 # => false
Integer === 42 # => true
Integer === 'fourtytwo' # => false
/ell/ === 'Hello' # => true
/ell/ === 'Foobar' # => false
除了上面说的 User === User.first 以外，还有 User.first.is_a? User 可以用。这里当然也可以 User.first.class == User。
=== 不是判断相等的运算符。
[1]: https://stackoverflow.com/questions/4467538/what-does-the-operator-do-in-ruby
=== 等价于 case when 中的条件判断：
condition = 'hello'
case condition
when String
when User
When (1...10)
when /ello/
end
更侧重归属感。
我觉得在 ruby 中要回答这个问题，应该是说，看这个 ===的定义
#User.first.class.method(:===).source_location
# "/Users/qinmingyuan/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/activerecord-5.2.0/lib/active_record/core.rb" 250L
# Overwrite the default class equality method to provide support for decorated models.
def ===(object)
object.is_a?(self)
end
约定俗称的定义都没有源码靠谱