module ActiveRecord::ReadonlyAttributes::ClassMethods

post.update(title: "a different title") # change to title will be ignored
post = Post.create!(title: "Introducing Ruby on Rails!")

end
attr_readonly :title
class Post < ActiveRecord::Base

==== Examples

You can assign a new value to a readonly attribute, but it will be ignored when the record is updated.

ignore these fields.
Attributes listed as readonly will be used to create a new record but update operations will

def attr_readonly(*attributes)
  self._attr_readonly = Set.new(attributes.map(&:to_s)) + (_attr_readonly || [])
end