What is the right way to override a setter method in Ruby on Rails?

In rails 4

let say you have age attribute in your table

def age=(dob)   
    now = Time.now.utc.to_date
    age = now.year - dob.year - ((now.month > dob.month || (now.month == dob.month && now.day >= dob.day)) ? 0 : 1)
    super(age) #must add this otherwise you need to add this thing and place the value which you want to save. 
  end

Note: For new comers in rails 4 you don't need to specify attr_accessible in model. Instead you have to white-list your attributes at controller level using permit method.

Use the super keyword:

def attribute_name=(value)
  super(value.some_custom_encode)
end

Conversely, to override the reader:

def attribute_name
  super.some_custom_decode
end

=========================================================================== Update: July 19, 2017

Now the Rails documentation is also suggesting to use super like this:

class Model < ActiveRecord::Base

  def attribute_name=(value)
    # custom actions
    ###
    super(value)
  end

end

===========================================================================

Original Answer

If you want to override the setter methods for columns of a table while accessing through models, this is the way to do it.

class Model < ActiveRecord::Base
  attr_accessible :attribute_name

  def attribute_name=(value)
    # custom actions
    ###
    write_attribute(:attribute_name, value)
    # this is same as self[:attribute_name] = value
  end

end

See Overriding default accessors in the Rails documentation.

So, your first method is the correct way to override column setters in Models of Ruby on Rails. These accessors are already provided by Rails to access the columns of the table as attributes of the model. This is what we call ActiveRecord ORM mapping.

Also keep in mind that the attr_accessible at the top of the model has nothing to do with accessors. It has a completely different functionlity (see this question)

But in pure Ruby, if you have defined accessors for a class and want to override the setter, you have to make use of instance variable like this:

class Person
  attr_accessor :name
end

class NewPerson < Person
  def name=(value)
    # do something
    @name = value
  end
end

This will be easier to understand once you know what attr_accessor does. The code attr_accessor :name is equivalent to these two methods (getter and setter)

def name # getter
  @name
end

def name=(value) #  setter
  @name = value
end

Also your second method fails because it will cause an infinite loop as you are calling the same method attribute_name= inside that method.