NSManagedObject实例的值copy

从官网的文档我们知道，实例化NSManagedObject不能像一般的nsobject一样[[NSManagedObjectalloc] init],这样运行时会出错，正确的方法是：<style type="text/css"> p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.0px Menlo; color: #008423} </style>

[NSEntityDescription insertNewObjectForEntityForName:entity inManagedObjectContext:[db managedObjectContext]] 或

[[NSManagedObject alloc]initWithEntity:entityinManagedObjectContext:[db managedObjectContext]]

返回的实例。

如果我们想把一个实列的值copy到另一个实例，如何做呢，我们可以根据属性，一个一个copy，但是代码不灵活，如果我们改了数据库字段，那么代码又要改，我发现了一个通用的代码。

- (NSManagedObject*)copyObject:(NSManagedObject*)object
                     toContext:(NSManagedObjectContext*)moc
                        parent:(NSString*)parentEntity;
{
  NSString *entityName = [[object entity] name];
 
  NSManagedObject *newObject = [NSEntityDescription
                                insertNewObjectForEntityForName:entityName
                                inManagedObjectContext:moc];
  [[self lookup] setObject:newObject forKey:[object objectID]];
 
  NSArray *attKeys = [[[object entity] attributesByName] allKeys];
  NSDictionary *attributes = [object dictionaryWithValuesForKeys:attKeys];
 
  [newObject setValuesForKeysWithDictionary:attributes];
 
  id oldDestObject = nil;
  id temp = nil;
  NSDictionary *relationships = [[object entity] relationshipsByName];
  for (NSString *key in [relationships allKeys]) {
   
    NSRelationshipDescription *desc = [relationships valueForKey:key];
    NSString *destEntityName = [[desc destinationEntity] name];
    if ([destEntityName isEqualToString:parentEntity]) continue;
   
    if ([desc isToMany]) {
     
      NSMutableSet *newDestSet = [NSMutableSet set];
     
      for (oldDestObject in [object valueForKey:key]) {
        temp = [[self lookup] objectForKey:[oldDestObject objectID]];
        if (!temp) {
          temp = [self copyObject:oldDestObject
                        toContext:moc
                           parent:entityName];
        }
        [newDestSet addObject:temp];
      }
     
      [newObject setValue:newDestSet forKey:key];
     
    } else {
      oldDestObject = [object valueForKey:key];
      if (!oldDestObject) continue;
     
      temp = [[self lookup] objectForKey:[oldDestObject objectID]];
      if (!temp && ![destEntityName isEqualToString:parentEntity]) {
        temp = [self copyObject:oldDestObject
                      toContext:moc
                         parent:entityName];
      }
     
      [newObject setValue:temp forKey:key];
    }
  }
 
  return newObject;
}

源码来自：http://pastebin.com/efkji4sy

ebook：http://pragprog.com/book/mzcd/core-data