title: Core Data Classes vs Model
body: <p>I've recently started down the road of programming in Objective-C, and I'm now looking into Core Data.  However, I am confused about exactly what defines the model itself in Core Data.</p>
  
  <p>Here's what I mean:  Say I create an entity with some set of attributes using the graphical model builder.  I then have Xcode generate code for the corresponding class.  Next, I want to create a property in the class that will be used only during run-time and does not need to be stored or retrieved by Core Data.  So, I add a variable and a corresponding property to the class (synthesizing it in the implementation) </p>
  
  <p>The new property is not defined in the model builder, but it is defined in the class derived from NSManagedObject.  How is it treated  in Core Data?  Do the properties listed in the class define attributes in the "model" or do only the attributes defined in the model builder define the model?</p>
  
  <p>Similarly, I wanted to add a enum-based property to the class file that, when get or set, accesses or changes an NSNumber attribute in the model.   Can I do that without Core Data treating the property as an attribute to be stored and retrieved?</p>
  
  <p>THANKS!</p>