self 指向当前 class，所以这种定义方法会定义出 class 方法（class method），如果不加 self. will define the instance method.

This way of writing self. is equivalent to:

  def Category.last_updated_at
    ...
  end

There are two types of methods that a ruby ​​object can call.
One type is defined in the class of the object, called instance method, such as

def last_updated_at
    #blahblah
end

Then the instance of Category can call this method, but the Category itself cannot

category1=Category.new
category2=Category.new
category1.last_updated_at    #legal
category2.last_updated_at    #legal
Category.last_updated_at    #illegal

The other type is called singleton method, which can only be called by one object, not the class of the object, nor other objects belonging to the same class as the object. The singleton method is defined as follows

#category1=Category.new
def category1.last_updated_at
    #blahblah
end

After this definition, only the category1 object can call the last_updated_at method

category1.last_updated_at    #legal
category2=Category.new
category2.last_updated_at    #illegal
Category.last_updated_at    #illegal

Ruby classes are also objects, and they can also define their own singleton methods, such as

def Category.last_udpated_at
    #blahblah
end

In this way, only Category can call the last_updated_at method, but not its instances, nor other classes, nor any other objects to be exact

Category.last_updated_at    #legal
category=Category.new
category.last_updated_at    #illegal

Inside the definition of the Category class (outside the method), self refers to the Category itself, so

def Category.last_udpated_at
    #blahblah
end

can be replaced by

def self.last_udpated_at
    #blahblah
end

This is the method defined in your question. Do you understand now? In essence, it is the singleton method of Category, and Category is a class. The singleton method of a class is also called the class method of the class and can only be called by the class itself.

It should be noted that a class method is a singleton method, but it is a singleton method of a class. Other than that, there is nothing special about it. Just like the instance method is held by the class to which the object belongs, the singleton method is also held by something called the eigenclass of the object. There is a lot more knowledge about eigenclass, so I won’t mention it here. I will talk about it in the article.

Add self to define a class method, otherwise it is an instance method

Define class methods can also be

class << self
    def last_updated_at
    order('updated_at desc').limit(1).pluck(:updated_at).first.to_i
  end
end