what does "class << self" mean in Rails? [duplicate]

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class << self idiom in Ruby

Can someone please explain class << self to me?

I would like to know what does class << self statement mean in 开发者_Python百科a model class? And how does the statement inside it differ from those outside from it. For example:

class Post < ActiveRecord::Base

  class << self
     def search(q)
          # search from DB
     end
  end
   def search2(qq)
         # search from DB
   end
end

What does class << self mean?

What are the differences between method search(q) and search2(qq) ?

---

That is the same as

class Post < ActiveRecord::Base

  def self.search(q)
    # Class Level Method
    # search from DB
  end

  def search2(qq)
    # Instance Level Method
    # search from DB
  end
end

Class methods work on the class (e.g. Post), instance methods works on instances of that class (e.g. Post.new)

Some people like the class << self; code; end; way because it keeps all class level methods in a nice block and in one place.

Others like to prefix each method with self. to explicitly know that is a class method not an instance method. It's a matter of style and how you code. If you put all class methods in a block like class << self, and this block is long enough, the class << self line might be out of your editor view making it difficult to know that you are in the class instance block.

On the other hand, prefixing each method with self. and intermixing those with instance methods is also a bad idea, how do you know all the class methods while reading your code.

Pick an idiom which you prefer for your own code base but if you work on an open source project or you collaborate on someone else's code, use their code formatting rule.

---

It creates class methods as opposed to instance methods. It's the same as doing def self.search(q). Those two methods would be called like:

Post.search(q)
Post.new.search(qq)

---

search2 is an instance-method while search is a class-method. The class << self syntax enables you to group class methods below. There are three ways of defining class methods in Ruby:

class MyClass
    def self.method
        # do sth.
    end

    def MyClass.method2
        # do sth.
    end

    class << self
        def method3
            # do sth.
        end 

        def another_class_method
            # do sth.
        end
    end
end

Class-methods are called on a class, not on an instance. It is personal preference which of the three idioms you want to use. I prefer def self.method for no particular reason.