从现有的数据库 ID 关系加载 Core Data 关系

Question

我有一个应用程序，它在表格中包含大量数据，我正在编写一个获取一小部分数据的移动应用程序。表中的数据使用 ID 来表示关系，我正在尝试将其加载到 Core Data 模型中并保留关系。

但似乎没有一种简单的方法可以告诉 Core Data，“对于这个关系，我想要另一个表的外键。”相反，我有这个怪物（这是六个异步 RestKit 查询中的一个的简化版本，这些查询将返回填充数据库）：

[manager postObject:nil path:@"api/shiftservice/get" parameters:@{@"workerIds": @[@1]} success:^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult) {
    NSLog(@"Loaded shifts: %d", [[mappingResult array] count]);
    NSManagedObjectContext *context = [AppDelegate managedObjectContext];

    NSSet *positions = [context fetchObjectsForEntityName:@"NWPosition"];
    NSDictionary *positionDict = [NSMutableDictionary new];
    for (NWPosition *position in positions) [positionDict setValue:position forKey:position.positionId.stringValue];

    NSSet *workers = [context fetchObjectsForEntityName:@"NWWorker"];
    NSDictionary *workerDict = [NSMutableDictionary new];
    for (NWWorker *worker in workers) [workerDict setValue:worker forKey:worker.workerId.stringValue];

    NSSet *shifts = [context fetchObjectsForEntityName:@"NWShift"];
    for (NWShift *shift in shifts)
    {
        NWPosition *position = [positionDict valueForKey:shift.positionId.stringValue];
        position.shifts = [context fetchObjectsForEntityName:@"NWShift" withPredicateFormat:@"positionId = %d", position.positionId];
        shift.position = position;

        NSSet *tradesAsGetShift = [context fetchObjectsForEntityName:@"NWTrade" withPredicateFormat:@"getShiftId = %@", shift.shiftId];
        for (NWTrade *trade in tradesAsGetShift) trade.getShift = shift;
        shift.tradesAsGetShift = tradesAsGetShift;

        NSSet *tradesAsGiveShift = [context fetchObjectsForEntityName:@"NWTrade" withPredicateFormat:@"giveShiftId = %@", shift.shiftId];
        for (NWTrade *trade in tradesAsGiveShift) trade.giveShift = shift;
        shift.tradesAsGiveShift = tradesAsGiveShift;

        NWWorker *worker = [workerDict valueForKey:shift.workerId.stringValue];
        worker.shifts = [context fetchObjectsForEntityName:@"NWShift" withPredicateFormat:@"workerId = %d", worker.workerId];
        shift.worker = worker;
    }
} failure:^(RKObjectRequestOperation *operation, NSError *error) {
    NSLog(@"Failed to load shifts with error: %@", [error localizedDescription]);
}];

我在 http://www.cocoawithlove.com/2008/03/core-data-one-line-fetch.html 使用 Matt Gallagher 的 One-Line Fetch 的修改版本作为 fetchObjectsForEntityName。

无论如何，这对我来说似乎很可怕，就像我错过了一些明显的东西。有没有办法告诉 Core Data 数据库风格的外键？如果没有，是否有一种简单的方法来填充它们？因为为每一个实体做这么多的获取肯定看起来不是正确的方法。如果 RestKit 在这方面有所帮助，那就更好了。

Answer 1

不，但应该不需要设置反向关系。

Answer 2

感谢 Wain 和他富有洞察力的评论，我设法让 RestKit 以一种相当优雅的方式为我做这件事。这是我设置外键关系的方法。我设置了每个关系的两端，以便任何一组数据都可以先进入，并且关系仍然可以正确填充。

首先，我设置了管理器，告诉它使用 JSON 序列化：

RKManagedObjectStore *store = [AppDelegate managedObjectStore];

RKObjectManager *manager = [[RKObjectManager alloc] initWithHTTPClient:oauthClient];
[RKObjectManager setSharedManager:manager];
[RKMIMETypeSerialization registerClass:[RKNSJSONSerialization class] forMIMEType:RKMIMETypeJSON];
[manager.HTTPClient registerHTTPOperationClass:[AFJSONRequestOperation class]];
[manager registerRequestOperationClass:[AFJSONRequestOperation class]];
[manager setAcceptHeaderWithMIMEType:RKMIMETypeJSON];
[manager setRequestSerializationMIMEType:RKMIMETypeJSON];
manager.managedObjectStore = store;

接下来，我设置连接和响应描述符：

NSManagedObjectContext *context = [AppDelegate managedObjectContext];
NSDictionary *orgRels = [[NSEntityDescription entityForName:@"NWOrg" inManagedObjectContext:context] relationshipsByName];
NSDictionary *positionRels = [[NSEntityDescription entityForName:@"NWPosition" inManagedObjectContext:context] relationshipsByName];
// ...

NSIndexSet *statusCodes = RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful);

NSArray *orgConnections =
@[[self connectionForRelation:@"positions" localKey:@"orgId" foreignKey:@"orgId" withRels:orgRels],
  [self connectionForRelation:@"workers" localKey:@"orgId" foreignKey:@"orgId" withRels:orgRels]];
[manager addResponseDescriptor:[RKResponseDescriptor responseDescriptorWithMapping:
    [self entityMappingForName:@"NWOrg" fromDictionary:@{@"orgId": @"orgId", @"orgName": @"orgName"} withKey:@"orgId" andConnections:orgConnections]
    method:RKRequestMethodAny pathPattern:@"api/orgservice/get" keyPath:nil statusCodes:statusCodes]];

NSArray *positionConnections =
@[[self connectionForRelation:@"org" localKey:@"orgId" foreignKey:@"orgId" withRels:positionRels],
  [self connectionForRelation:@"shifts" localKey:@"positionId" foreignKey:@"positionId" withRels:positionRels]];
[manager addResponseDescriptor:[RKResponseDescriptor responseDescriptorWithMapping:
    [self entityMappingForName:@"NWPosition" fromDictionary:@{@"positionId": @"positionId", @"orgId": @"orgId", @"name": @"positionName"} withKey:@"positionId" andConnections:positionConnections]
    method:RKRequestMethodAny pathPattern:@"api/positionservice/get" keyPath:nil statusCodes:statusCodes]];

// ...

然后，我能够完成简单的请求，并且效果很好：

[manager postObject:nil path:@"api/shiftservice/get" parameters:@{@"workerIds": @[@1]}
    success:^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult) {
    NSLog(@"Loaded shifts: %d", [[mappingResult array] count]);
} failure:^(RKObjectRequestOperation *operation, NSError *error) {
    NSLog(@"Failed to load shifts with error: %@", [error localizedDescription]);
}];

注意：这使用了一些辅助方法，我将在此处重现：

- (RKConnectionDescription *)connectionForRelation:(NSString *)relation
    localKey:(NSString *)localKey foreignKey:(NSString *)foreignKey 
    withRels:(NSDictionary *)rels
{
    return [[RKConnectionDescription alloc] initWithRelationship:rels[relation]
        attributes:@{localKey: foreignKey}];
}

- (RKEntityMapping *)entityMappingForName:(NSString *)name
    fromDictionary:(NSDictionary *)dict withKey:(NSString *)key
{
    RKEntityMapping *mapping = [RKEntityMapping mappingForEntityForName:name
        inManagedObjectStore:[AppDelegate managedObjectStore]];
    [mapping addAttributeMappingsFromDictionary:dict];
    mapping.assignsDefaultValueForMissingAttributes = YES;
    mapping.assignsNilForMissingRelationships = YES;
    mapping.identificationAttributes = @[key];

    return mapping;
}

- (RKEntityMapping *)entityMappingForName:(NSString *)name
    fromDictionary:(NSDictionary *)dict withKey:(NSString *)key
    andConnections:(NSArray *)connections
{
    RKEntityMapping *mapping = [self entityMappingForName:name
        fromDictionary:dict withKey:key];
    for (RKConnectionDescription *connection in connections)
    {
        [mapping addConnection:connection];
    }
    return mapping;
}