在Ruby中，在类方法中，自我是类还是实例？

I know that self is the instance inside of an instance method. So, then, is self the class inside of a class method? E.g., Will the following work in Rails?

我知道self是实例方法中的实例。那么，在类方法中是自我类吗？例如，以下是否会在Rails中工作？

class Post < ActiveRecord::Base
  def self.cool_post
    self.find_by_name("cool")
  end
end

4 个解决方案

#1

That is correct. self inside a class method is the class itself. (And also inside the class definition, such as the self in def self.coolpost.)

那是对的。类方法中的self是类本身。（还有在类定义中，例如def self.coolpost中的self。）

You can easily test these tidbits with irb:

您可以使用irb轻松测试这些花絮：

class Foo
  def self.bar
    puts self.inspect
  end
end

Foo.bar  # => Foo

#2

class Test
    def self.who_is_self
        p self
    end
end

Test.who_is_self

output:

输出：

Test

测试

Now if you want a Rails specific solution, it's called named_scopes:

现在，如果你想要一个Rails特定的解决方案，它叫做named_scopes：

class Post < ActiveRecord::Base
   named_scope :cool, :conditions => { :name => 'cool' }
end

Used like this:

像这样使用：

Post.cool

#3

A lot of answers already, but here is why self is the class:

已经有很多答案，但这就是为什么自我是班级的原因：

The dot changes self to whatever is before the dot. So, when you do foo.bar then for the bar-method, self is foo. There is no difference with class methods. When calling Post.cool_post, you'll change self to Post.

点会将自身更改为点之前的任何内容。所以，当你执行foo.bar然后使用bar方法时，self就是foo。类方法没有区别。在调用Post.cool_post时，您将自己更改为Post。

The important thing to note here is that it's not how the method is defined that determines self, but how it's called. This is why this works:

这里要注意的重要一点是，不是如何定义方法来决定自我，而是如何调用它。这就是为什么这有效：

class Foo
  def self.bar
    self
  end
end

class Baz < Foo
end

Baz.bar # => Baz

Or this:

或这个：

module Foo
  def bar
    self
  end
end

class Baz
  extend Foo
end

Baz.bar # => Baz

#4

Short answer: yes

简短回答：是的

What I like to do with these questions is just fire up an irb or ./script/console session

我喜欢这些问题只是启动irb或./script/console会话

and then you can do the following to see the magic:

然后你可以做以下事情来看魔术：

ruby-1.8.7-p174 > class TestTest
ruby-1.8.7-p174 ?>  def self.who_am_i
ruby-1.8.7-p174 ?>    return self
ruby-1.8.7-p174 ?>    end
ruby-1.8.7-p174 ?>  end
 => nil 
ruby-1.8.7-p174 > TestTest.who_am_i
 => TestTest

Happy fishing!

快乐钓鱼！

#1