title: how to avoid polymorphic associations
body: <p>Given you have to implement a news feed like the one seen in social networks, ex facebook. 
  Currently I'm using a News class which has a polymorphic assocation which can be of any kind like Image, Comment, Friendship, GroupMembership, etc. Whenever an Object is created, as News is created too. It's working fine with AR(ActiveRecords) but I get into trouble when I'd switch to DM(DataMapper) or Sequel as both don't natevily support polymorphic associations and discourage it's usage. </p>
  
  <p>One workaround would be to use a big SQL clause with lot's of UNIONs to merge all the different tables which should be considered as news. But this has some drawbacks, escpecially performance would be terrible. </p>
  
  <p>So I wonder how to solve without polymorphic associations while still getting good performance and no other drawbacks, like having the possibility of adding meta data to a news?</p>