`alias_method_chain': undefined method `find' for class `ActiveRecord::Base' (NameError)

I can't seem to be able to override the :find method in the model base
class, for example, in /lib/usermonitor.rb:

module ActiveRecord
  module UserMonitor
    def self.included(base)
      base.class_eval do
        alias_method_chain :find, :user

        def current_site
          Thread.current[:user]
        end
      end
    end
    def find_with_user(*args)
      ...
    end
  end
end

I got this error:

`alias_method_chain': undefined method `find' for class
`ActiveRecord::Base' (NameError)

However, overriding :create and :update methods work fine. Why?