NSManagedObject实例的值copy

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

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

[[NSManagedObject alloc] initWithEntity:entity inManagedObjectContext:[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