NSEntityMigrationPolicy erstellt ein neues Objekt anstelle einer bestehenden Änderung

Question

Ich verwende die Methode NSEntityMigrationPolicy und createDestinationInstancesForSourceInstance, um einen Attributtyp von Data in String zu ändern. Es erzeugt jedoch neue Objekte, anstatt die vorhandene zu ersetzen.

Hier ist mein Code:

#import "MessageTransformationPolicy.h" 

@implementation MessageTransformationPolicy 

- (BOOL)createDestinationInstancesForSourceInstance:(NSManagedObject *)sInstance 
             entityMapping:(NSEntityMapping *)mapping 
              manager:(NSMigrationManager *)manager 
               error:(NSError **)error 
{ 
    // Create a new object for the model context 
    NSManagedObject *newObject = 
    [NSEntityDescription insertNewObjectForEntityForName:[mapping destinationEntityName] 
            inManagedObjectContext:[manager destinationContext]]; 

    // do our transfer of nsdate to nsstring 
    NSData *messageMetadata_dataType = [sInstance valueForKey:@"metadata"]; 
    NSString *messageMetadata_stringType = [[NSString alloc] initWithData:messageMetadata_dataType encoding:NSUTF8StringEncoding]; 

    // set the value for our new object 
    [newObject setValue:messageMetadata_stringType forKey:@"metadata"]; 

    // do the coupling of old and new 
    [manager associateSourceInstance:sInstance withDestinationInstance:newObject forEntityMapping:mapping]; 

    return YES; 
} 

@end 

Wo liegt das Problem?

Answer 1

Durch die Änderung der Art und Weise der Speicherung und Speichern aller Attribute der Zieleinheit Msg genannt, ist das Problem gelöst:

- (BOOL)createDestinationInstancesForSourceInstance:(NSManagedObject *)inSourceInstance 
             entityMapping:(NSEntityMapping *)inMapping 
              manager:(NSMigrationManager *)inManager 
               error:(NSError **)outError { 
    NSManagedObject *newObject; 
    NSEntityDescription *sourceInstanceEntity = [inSourceInstance entity]; 

    // sure to have the right entity 
    if ([[sourceInstanceEntity name] isEqualToString:@"Msg"]) { 
     newObject = [NSEntityDescription insertNewObjectForEntityForName:@"Msg" inManagedObjectContext:[inManager destinationContext]]; 

     // get the attributes 
     NSDictionary *keyValDict = [inSourceInstance committedValuesForKeys:nil]; 
     NSArray *allKeys = [[[inSourceInstance entity] attributesByName] allKeys]; 
     // loop over the attributes 
     for (NSString *key in allKeys) { 
      // Get key and value 
      id value = [keyValDict objectForKey:key]; 
      if ([key isEqualToString:@"metadata"]) {     
       NSData *messageMetadata_dataType = [keyValDict valueForKey:@"metadata"]; 
       NSString *messageMetadata_stringType = [[NSString alloc] initWithData:messageMetadata_dataType encoding:NSUTF8StringEncoding]; 

       [newObject setValue:messageMetadata_stringType forKey:key]; 
      } else { 
       [newObject setValue:value forKey:key]; 
      } 
     } 

     [inManager associateSourceInstance:inSourceInstance withDestinationInstance:newObject forEntityMapping:inMapping]; 
    } 
    return YES; 
}