title: CoreData - Store one entity as 'active'
body: <p>Given a group of core data entities (let's call them phrases), what is the best/most correct way of having one phrase as "the active phrase" and enforcing only one active phrase at a given time.</p>
  
  <p>I thought about having a Bool value on the phrase entity, but I don't know how to enforce <code>true</code> to only be on one entity out of the set. I guess I could also have another entity (activePhrase) which stores just one record in it but again I'm not sure how to enforce the 'select one of many phrases to be active' requirement. </p>
  
  <p>I fear this may be a dumb question - my CoreData (and indeed database) knowledge is a bit rusty and I want to make sure I'm following best steps. </p>