log_model_updates.rb

module LogModelUpdates
  def self.included(base)
    base.send :include, InstanceMethods
  end

  module InstanceMethods
    attr_accessor :current_user
    attr_accessor :row_updates

    def before_save
      log_row_updates
    end

    def after_save
      create_row_updates!
    end

    def log_row_updates
      @row_updates = []
      update_type = (self.new_record? ? "insert" : "update")
      if update_type == "insert"
          @row_updates << RowUpdate.new(  :user_id => @current_user,
                                          :row_id => (self.id || nil),
                                          :update_type => update_type,
                                          :table_name => self.class.table_name.to_s )
      else
          self.changes.each do |attr|
              @row_updates << RowUpdate.new(  :user_id => @current_user.id,
                                              :row_id => (self.id || nil),
                                              :update_type => update_type,
                                              :table_name => self.class.table_name.to_s,
                                              :column_name => attr[0],
                                              :before_value => attr[1][0],
                                              :after_value => attr[1][1]  )
          end
      end
    end

    def create_row_updates!
      @row_updates.each do |row|
        row.row_id = self.id if row.row_id.nil?
        begin
          row.save!
        rescue
          # TODO: Handle failed insert
        end
      end
    end
  end
end

The issue had to deal with the callbacks not being instantiated properly. The following code fixed it:

def self.extended(base)
base.before_save :log_row_updates
base.after_save :create_row_updates!
base.send :include, InstanceMethods
end