Restkit - 只有子 ID 的映射答案

【问题标题】：Restkit - mapping having only child idRestkit - 只有子 ID 的映射
【发布时间】：2013-01-16 10:55:08
【问题描述】：

我正在尝试在我的 iOS 应用程序中使用 RestKit 0.20 将 json 映射到核心数据对象。我有两种类型的对象：用户和消息。用户有很多消息。消息具有指向用户的字段：'from' 和 'to'。这是我的消息 json：

[
    {
        "created_at":1358265482867,
        "to":"50ed7b4e63e9b80200000009",
        "note":"Hello",
        "from":"50ee98365315d00200000003",
        "_id":"50f57cee11f2b00200000016",
        "__v":0
    },
    {
        "created_at":1358265582867,
        "to":"50ed7b4e63e9b80200000009",
        "note":"Hello 2",
        "from":"50ee98365315d00200000003",
        "_id":"50f57cee11f2b00200000016",
        "__v":0
    }
]

我的映射代码如下：

// Object mappings
RKEntityMapping *userMapping = [RKEntityMapping mappingForEntityForName:@"User" inManagedObjectStore:managedObjectStore];
userMapping.identificationAttributes = @[@"userId"];
[userMapping addAttributeMappingsFromDictionary:@{
     @"_id": @"userId",
     @"name": @"name",
     @"note": @"headline",
     @"photo_url": @"photoUrl"
 }];

RKEntityMapping *messageMapping = [RKEntityMapping mappingForEntityForName:@"Message" inManagedObjectStore:managedObjectStore];
messageMapping.identificationAttributes = @[@"messageId"];
[messageMapping addAttributeMappingsFromDictionary:@{
     @"_id": @"messageId",
     @"note": @"note",
     @"created_at": @"createdAt",
     @"from": @"from",
     @"to" : @"to"
 }];

// !!! This code is ok if json contains entire object (but contains only id).
// [messageMapping addPropertyMapping:[RKRelationshipMapping relationshipMappingFromKeyPath:@"from" toKeyPath:@"from" withMapping:userMapping]];
// [messageMapping addPropertyMapping:[RKRelationshipMapping relationshipMappingFromKeyPath:@"to" toKeyPath:@"to" withMapping:userMapping]];

RKEntityMapping *messageReversedMapping = [messageMapping inverseMapping];
RKRequestDescriptor *requestDescription = [RKRequestDescriptor requestDescriptorWithMapping:messageReversedMapping objectClass:[Message class] rootKeyPath:nil];
[objectManager addRequestDescriptor:requestDescription];

RKResponseDescriptor *userResponseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping:userMapping pathPattern:@"/users" keyPath:nil statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)];
[objectManager addResponseDescriptor:userResponseDescriptor];

RKResponseDescriptor *messageResponseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping:messageMapping pathPattern:@"/messages" keyPath:nil statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)];
[objectManager addResponseDescriptor:messageResponseDescriptor];

型号代码：

@interface Message : NSManagedObject

@property (nonatomic, retain) NSString * note;
@property (nonatomic, retain) NSNumber * createdAt;
@property (nonatomic, retain) NSString * messageId;
@property (nonatomic, retain) User *from;
@property (nonatomic, retain) User *to;


@interface User : NSManagedObject

@property (nonatomic, retain) NSString * email;
@property (nonatomic, retain) NSString * headline;
@property (nonatomic, retain) NSString * lastName;
@property (nonatomic, retain) NSNumber * me;
@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSString * photoUrl;
@property (nonatomic, retain) NSString * userId;
@property (nonatomic, retain) Message *messagesFrom;
@property (nonatomic, retain) Message *messagesTo;

问题在于，在 json 中，字段“to”和“from”是用户的 id，而不是嵌套对象，因此应用程序崩溃。

我发现了类似的 SO 问题，但仍然无法正常工作。

Mapping nested IDs with RestKit in Xcode using Objective-C

Mapping relationships in RestKit through an array of IDs doesn't work

谁能帮我正确设置映射，这样 json 将只包含用户的 ID 而不是嵌套对象？

【问题讨论】：

标签： objective-c restkit

【解决方案1】：

好的，我想通了。 API 文档给出了如何映射这种关系的示例： http://restkit.org/api/latest/Classes/RKConnectionDescription.html

我仍然需要将字段 'from' 和 'to' 映射到虚拟的 'fromUserId' 和 'toUserId' 但它可以工作。

此代码适用于我：

NSManagedObjectContext *context = ApplicationDelegate.managedObjectContext;
NSEntityDescription *messageEntity = [NSEntityDescription entityForName:@"Message" inManagedObjectContext:context];
NSRelationshipDescription *userRelationshipFrom = [messageEntity relationshipsByName][@"from"];
RKConnectionDescription *connectionUserMessageFrom = [[RKConnectionDescription alloc] initWithRelationship:userRelationshipFrom attributes:@{ @"fromUserId": @"userId" }];
[messageMapping addConnection:connectionUserMessageFrom];

NSRelationshipDescription *userRelationshipTo = [messageEntity relationshipsByName][@"to"];
RKConnectionDescription *connectionUserMessageTo = [[RKConnectionDescription alloc] initWithRelationship:userRelationshipTo attributes:@{ @"toUserId": @"userId" }];
[messageMapping addConnection:connectionUserMessageTo];

【讨论】：