How can I duplicate, or copy a Core Data Managed Object?

Question

I have a managed object ("A") that contains various attributes and types of relationships, and its relationships also have their own attributes & relationships. What I would like to do is to "copy" or "duplicate" the entire object graph rooted at object "A", and thus creating a new object "B" that is very similar to "A".

To be more specific, none of the relationships contained by "B" (or its children) should point to objects related to "A". There should be an entirely new object graph with similar relationships intact, and all objects having the same attributes, but of course different id's.

There is the obvious manual way to do this, but I was hoping to learn of a simpler means of doing so which was not totally apparent from the Core Data documentation.

TIA!

Answer 1

These answers got me really close, though they did seem to have some shortcomings:

1st, I took the advice of Z S and made it a category on NSManagedObject, this seemed a little cleaner to me.

2nd, My object graph contains to-one relationships, so I started from levous's example, but note that levous's example isn't cloning the object in the case of the to-one relationship. This will cause a crash (attempting to save a NSMO from one context in a different context). I have addressed that in the example below.

3rd, I provided a cache of already-cloned objects, this prevents objects from being cloned twice and therefore duplicated in the new object graph, and also prevents cycles.

4th, I've added a black-list (list of Entity-types not to clone). I did this in part to solve one shortcoming of my final solution, which I will describe below.

NOTE: if you use what I understand to a be CoreData best-practice, always providing inverse relationships, then this will likely clone all objects that have a relationship to the object you want to clone. If you are using inverses and you have a single root object that knows about all other objects, then you will likely clone the whole thing. My solution to this was to add the blacklist and pass in the Entity type that I knew was a parent of one of the objects I wanted cloned. This appears to work for me. :)

Happy cloning!

// NSManagedObject+Clone.h #import <CoreData/CoreData.h>  @interface NSManagedObject (Clone) - (NSManagedObject *)cloneInContext:(NSManagedObjectContext *)context exludeEntities:(NSArray *)namesOfEntitiesToExclude; @end   // NSManagedObject+Clone.m #import "NSManagedObject+Clone.h"  @implementation NSManagedObject (Clone)  - (NSManagedObject *)cloneInContext:(NSManagedObjectContext *)context withCopiedCache:(NSMutableDictionary *)alreadyCopied exludeEntities:(NSArray *)namesOfEntitiesToExclude {   NSString *entityName = [[self entity] name];    if ([namesOfEntitiesToExclude containsObject:entityName]) {     return nil;   }    NSManagedObject *cloned = [alreadyCopied objectForKey:[self objectID]];   if (cloned != nil) {     return cloned;   }    //create new object in data store   cloned = [NSEntityDescription insertNewObjectForEntityForName:entityName inManagedObjectContext:context];   [alreadyCopied setObject:cloned forKey:[self objectID]];    //loop through all attributes and assign then to the clone   NSDictionary *attributes = [[NSEntityDescription entityForName:entityName inManagedObjectContext:context] attributesByName];    for (NSString *attr in attributes) {     [cloned setValue:[self valueForKey:attr] forKey:attr];   }    //Loop through all relationships, and clone them.   NSDictionary *relationships = [[NSEntityDescription entityForName:entityName inManagedObjectContext:context] relationshipsByName];   for (NSString *relName in [relationships allKeys]){     NSRelationshipDescription *rel = [relationships objectForKey:relName];      NSString *keyName = rel.name;     if ([rel isToMany]) {       //get a set of all objects in the relationship       NSMutableSet *sourceSet = [self mutableSetValueForKey:keyName];       NSMutableSet *clonedSet = [cloned mutableSetValueForKey:keyName];       NSEnumerator *e = [sourceSet objectEnumerator];       NSManagedObject *relatedObject;       while ( relatedObject = [e nextObject]){         //Clone it, and add clone to set         NSManagedObject *clonedRelatedObject = [relatedObject cloneInContext:context withCopiedCache:alreadyCopied exludeEntities:namesOfEntitiesToExclude];         [clonedSet addObject:clonedRelatedObject];       }     }else {       NSManagedObject *relatedObject = [self valueForKey:keyName];       if (relatedObject != nil) {         NSManagedObject *clonedRelatedObject = [relatedObject cloneInContext:context withCopiedCache:alreadyCopied exludeEntities:namesOfEntitiesToExclude];         [cloned setValue:clonedRelatedObject forKey:keyName];       }     }   }    return cloned; }  - (NSManagedObject *)cloneInContext:(NSManagedObjectContext *)context exludeEntities:(NSArray *)namesOfEntitiesToExclude {   return [self cloneInContext:context withCopiedCache:[NSMutableDictionary dictionary] exludeEntities:namesOfEntitiesToExclude]; }  @end 

Answer 2

Here's a class I created to perform a "deep copy" of managed objects: attributes and relationships. Note that this does not check against loops in the object graph. (Thanks Jaanus for the starting off point...)

@interface ManagedObjectCloner : NSObject { } +(NSManagedObject *)clone:(NSManagedObject *)source inContext:(NSManagedObjectContext *)context; @end  @implementation ManagedObjectCloner  +(NSManagedObject *) clone:(NSManagedObject *)source inContext:(NSManagedObjectContext *)context{     NSString *entityName = [[source entity] name];      //create new object in data store     NSManagedObject *cloned = [NSEntityDescription                                insertNewObjectForEntityForName:entityName                                inManagedObjectContext:context];      //loop through all attributes and assign then to the clone     NSDictionary *attributes = [[NSEntityDescription                                  entityForName:entityName                                  inManagedObjectContext:context] attributesByName];      for (NSString *attr in attributes) {         [cloned setValue:[source valueForKey:attr] forKey:attr];     }      //Loop through all relationships, and clone them.     NSDictionary *relationships = [[NSEntityDescription                                    entityForName:entityName                                    inManagedObjectContext:context] relationshipsByName];     for (NSRelationshipDescription *rel in relationships){         NSString *keyName = [NSString stringWithFormat:@"%@",rel];         //get a set of all objects in the relationship         NSMutableSet *sourceSet = [source mutableSetValueForKey:keyName];         NSMutableSet *clonedSet = [cloned mutableSetValueForKey:keyName];         NSEnumerator *e = [sourceSet objectEnumerator];         NSManagedObject *relatedObject;         while ( relatedObject = [e nextObject]){             //Clone it, and add clone to set             NSManagedObject *clonedRelatedObject = [ManagedObjectCloner clone:relatedObject                                                            inContext:context];             [clonedSet addObject:clonedRelatedObject];         }      }      return cloned; }   @end