class method self.attr_protected

Attributes named in this macro are protected from mass-assignment, such as new(attributes), update_attributes(attributes), or attributes=(attributes).

Mass-assignment to these attributes will simply be ignored, to assign to them you can use direct writer methods. This is meant to protect sensitive attributes from being overwritten by malicious users tampering with URLs or forms.

class Customer < ActiveRecord::Base
  attr_protected :credit_rating
end

customer = Customer.new("name" => David, "credit_rating" => "Excellent")
customer.credit_rating # => nil
customer.attributes = { "description" => "Jolly fellow", "credit_rating" => "Superb" }
customer.credit_rating # => nil

customer.credit_rating = "Average"
customer.credit_rating # => "Average"

To start from an all-closed default and enable attributes as needed, have a look at attr_accessible.