Rahul Katariya Rahul Katariya - 2 years ago 89
Swift Question

CoreData - Benefits of NSManagedObject Subclass

I am trying to insert into CoreData without creating a subclass of NSManagedObject. but my app crashes with NSManagedObject setValue:forUndefinedKey "name" in Category.

let managedObjectContext = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext
let entitiyDesc = NSEntityDescription()
entitiyDesc.name = "Category"
guard let data = model.data else { return }
for d in data {
let managedObject = NSManagedObject.init(entity: entitiyDesc, insertIntoManagedObjectContext: managedObjectContext)
managedObject.setValue(d.name, forKey: "name")
try! managedObjectContext.save()

What are the benefits of subclassing NSManagedObjects

Answer Source

There is nothing wrong with the way you are using Core Data. Generally you start wanting to subclass when:

  • You want convenience methods on your NSManagedObject
  • You are using transient values that are not in the model at all
  • Want to react to the creation or insertion of an NSManagedObject
  • Want to present an easier to use object to your UI developers
  • Want to avoid using "magic strings" when accessing properties

and I am sure there are more in that list.

You never need to subclass an NSManagedObject but in many situations it does make your code cleaner and easier to maintain.

A couple of comments on your code:

  • NSEntityDescription and NSManagedObject should not be created that way. You should be using the convenience method NSEntityDescription.insertNewObjectForEntityForName(_:) instead
  • Using a forced try like that is very bad for error handling. You are FAR better off using a do/catch so that you can interrogate the error completely. This is especially important with Core Data that will send you sub errors.
  • Accessing the NSManagedObjectContext through the AppDelegate like that is a poor design (yes I know its in the Apple template). It is far better to use dependency injection as discussed in the Core Data Programming Guide and avoid the tight coupling that is inherent with accessing the AppDelegate
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