To use self. or not.. in Rails

Question

I've been coding in Ruby for sometime now, but I don't understand when to use:

def self.METHOD_NAME end 

or just:

def METHOD_NAME end 

In any Rails model. Is "self" a modifier like private in Java? When should I use it and when not?. Thanks a ton.

Answer 1

def self.method_name end 

defines a class method.

def method_name end 

defines an instance method.

This is a pretty good post on it.

Answer 2

A quick explanation of what that means:

In ruby, you can define methods on a particular object:

a = "hello"  def a.informal   "hi" end  a.informal => "hi" 

What happens when you do that is that the object a, which is of class String, gets its class changed to a "ghost" class, aka metaclass, singleton class or eigenclass. That new class superclass is String.

Also, inside class definitions, self is set to the class being defined, so

class Greeting   def self.say_hello     "Hello"   end   #is the same as:   def Greeting.informal     "hi"   end end 

What happens there is that the object Greeting, which is of class Class, gets a new metaclass with the new methods, so when you call

Greeting.informal => "hi" 

There's no such thing as class methods in ruby, but the semantics are similar.