has_many:topics,:class_name=>'Memo',:conditions=>"#{Memo.table_name}.parent_id IS NULL",:order=>"#{Memo.table_name}.created_at DESC"
has_many:topics,:class_name=>'Memo',:conditions=>"#{Memo.table_name}.parent_id IS NULL",:order=>"#{Memo.table_name}.created_at DESC",:dependent=>:destroy