ruby - 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.