title: Filtering results in GraphQL using PostGraphile
body: <p>I'm trying to wrap my head around GraphQL and I though using PostGraphile to easily and quickly map my PostgreSQL database and expose it using GraphQL. However, I've been stuck a long time on some things that in simple SQL would be a matter of minutes to do -</p>
  
  <p>First, I'm trying to get all records from my database after a defined date, couldn't do this so far, and I end up getting <em>all</em> records which is highly inefficient.</p>
  
  <p>Second, I'd like to get all records which a nullable field in them isn't null (meaning, only if it has something in it, it will show up in the GraphQL results)</p>
  
  <p>If anyone could shed some light on how to do this, or point me to a good tutorial that explains in a simple way how to write custom filtering functions that would be great.</p>