NSEntityMigrationPolicy creates new object instead of change existing one - ios

Question

I use NSEntityMigrationPolicy and createDestinationInstancesForSourceInstance method to change one attribute type from Data to String. However it creates new objects instead of replacing the existing one.
Here is my 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
Where is the problem?

Answer 1

By changing the way of storing and saving all attributes of the target entity named Msg, the problem solved:
- (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;
}