I will try to give an outline of how it can be done , please edit this according to your requirement

in your view have a text field where users will be searching for a name.

'',

                                        :size => 40
                                      },
                                      {
                                        :controller           => '/grandparents',
                                        :frequency            => 0.5,                                           
                                        :min_chars            => 2
                                      }
                                     ) %>

In your controller your have to write a method that will capture the text typed and will query the database for a match.

def auto_complete_for_grandparents_name
  search = params[xxxxx]  # get the name typed in the text field
  @matches= GrandParent.find_by_sql("SELECT  id, name FROM grand_parents WHERE (name LIKE '%#{search.sql_safe}%')")     
end

You can use the render :inline property to display the result set in your view .