Can I create an new instance of my custom managed object class without going through NSEntityDescription?

Question

From an Apple example, I have this:

Event *event = (Event*)[NSEntityDescription 
    insertNewObjectForEntityForName:@"Event" 
             inManagedObjectContext:self.managedObjectContext];

Event inherits from NSManagedObject. Is there a way to avoid this weird call to NSEntityDescription and instead just alloc+init somehow directly the Event class? Would I have to write my own initializer that just does that stuff above? Or is NSManagedObject already intelligent enough to do that?

Answer 1

NSManagedObject provides a method called initWithEntity:insertIntoManagedObjectContext:. You can use this to do a more traditional alloc/init pair. Keep in mind that the object this returns is not autoreleased.

Answer 2

To get it to work properly, there is a LOT of stuff to do. -insertNewObject:... is by far the easiest way, weird or not. The documentation says:

A managed object differs from other objects in three main ways—a managed object ... Exists in an environment defined by its managed object context ... there is therefore a lot of work to do to create a new managed object and properly integrate it into the Core Data infrastructure ... you are discouraged from overriding initWithEntity:insertIntoManagedObjectContext:

That said, you can still do it (read further down the page to which I linked) but your goal appears to be "easier" or "less weird". I'd say the method you feel is weird is actually the simplest, most normal way.

Answer 3

I found a definitive answer from More iPhone 3 Development by Dave Mark and Jeff LeMarche.

If it really bothers you that you use a method on NSEntityDescrpiton rather than on NSManagedObjectContext to insert a new object into an NSManagedObjectContext, you can use a category to add an instance method to NSManagedObjectContext.

Create two new text files called NSManagedObject-Insert.h and NSManagedObject-Insert.m.

In NSManagedObject-Insert.h, place the following code:

import <Cocoa/Cocoa.h>
@interface NSManagedObjectContext (insert)
- (NSManagedObject *)insertNewEntityWithName:(NSString *)name;
@end

In NSManagedObject-Insert.m, place this code:

#import "NSManagedObjectContext-insert.h"

@implementation NSManagedObjectContext (insert)
- (NSManagedObject *)insertNewEntityWithName:(NSString *)name
{
return [NSEntityDescription insertNewObjectForEntityForName:name inManagedObjectContext:self];
}
@end

You can import NSManagedObject-Insert.h anywhere you wish to use this new method. Then replace the insert calls against NSEntityDescription, like this one:

NSManagedObject *newManagedObject = [NSEntityDescription insertNewObjectForEntityForName:[entity name] inManagedObjectContext:context];

with the shorter and more intuitive one:

[context insertNewEntityWithName:[entity name]];

Aren't categories grand?