Class Method in Ruby

In Ruby, a method provides functionality to an Object. A class method provides functionality to a class itself, while an instance method provides functionality to one instance of a class. In other word, a class method is a method that is called on the class itself, not on the instances of that class.

For example, let’s say we have a class, Person. Every individual person instance should have a name attribute. To accomplish this, we’ll define an instance variable, @name and an instance method #name that exposes or reveals that variable.

class Person

    def initialize(name)
        @name = name
    end

    def name=(name)
        @name = name
    end

    def name
        @name
    end

end

Here we have an instance variable, @name, which can be set equal to a value using the name=() method, a setter method. Then, we have a getter method name that returns the value of @name. Now, we can execute the following:

person = Person.new
person.name = "Nasrin"
person.name
# => "Nasrin" 

Let’s say we wanted to keep a track of people we had in our system. Now we should have a method in class scope not in instance scope.

class Person
    @@all = []

    def initialize(name)
        @name = name
        @@all << self
    end

    def name=(name)
        @name = name
    end

    def name
        @name
    end

    def self.all
        @@all
    end
end 

Now, whenever we initialize an instance a Person with Person.new, that person will add to @@all array, and Person.all returns that array.

nasrin = Person.new("Nasrin")
fariborz = Person.new("Fariborz")

Person.all #=> [#<Person @name="Nasrin">,
#<Person @name="Fariborz">] 

Ok, for finding a specific person by name given the Person model, we can define another class method like below:

def self.find_by_name(name)
    @@all.find{|person| person.name == name}
end

We call class methods like Person.find_by_name ‘finders’. Finder class methods are responsible for finding instances based on some property or condition.

To read more about class method in ruby, check below link:

https://www.geeksforgeeks.org/ruby-class-method-and-variables/

Thanks for reading!