title: Multiple Foreign Keys
body: <p>My database designs have previously been rather linear, so I'm getting stumped on what is probably a very easy to solve problem.</p>
  
  <p>I have a table of "POSTS", which contain posts which may be a child of either a "CATEGORY" or a "TOPIC".  What would be the best way to define the foreign key(s) for the "POSTS" table?</p>
  
  <p>I suppose I could have a column named POST_CATEGORY_ID and a field named "POST_TOPIC_ID, which could be nullable, but this just doesn't sound right.  Surely there is a simple solution that I'm missing!</p>