title: Core Data primary key ID for a row in the database
body: <p>Suppose I have a list of books stored in Core Data. I want to search for a book by it's primary key ID.</p>
  <p>I know the sqlite file created by Core Data has an ID column in each table, but this doesn't seem to be exposed to me in anyway.</p>
  <p>Does anyone have any recommendations?</p>