RestKit 多对多映射，怎么做？

Question

我正在开发我的第一个 RestKit 应用程序，我使用 CoreData 进行本地缓存。在其中，我在 Post > 类别之间建立了多对多的关系。这种关系称为帖子和类别。

在我的 Post JSON 调用中，我得到如下类别的 ID：{"post": [...] "categories":[1,4] [...]，其中 1 和 4 是 ID。

我有一个继承自 NSManagedObject 的自定义 Post 模型对象，其中我有一个类别属性，如下所示：

@interface Post : NSManagedObject

[...]
@property (nonatomic, retain) NSSet *categories;
[...]

我在自定义类别模型对象中做同样的事情。

我的映射目前如下所示：

RKManagedObjectMapping *categoryMapping = [RKManagedObjectMapping mappingForClass:[Category class] inManagedObjectStore:objectManager.objectStore];
categoryMapping.primaryKeyAttribute = @"categoryId";
categoryMapping.rootKeyPath = @"categories";
[categoryMapping mapKeyPath:@"id" toAttribute:@"categoryId"];
[categoryMapping mapKeyPath:@"name" toAttribute:@"name"];
[...]


RKManagedObjectMapping* postMapping = [RKManagedObjectMapping mappingForClass:[Post class] inManagedObjectStore:objectManager.objectStore];

postMapping.primaryKeyAttribute = @"postId";
postMapping.rootKeyPath = @"post";
[postMapping mapKeyPath:@"id" toAttribute:@"postId"];
[...]
[postMapping mapKeyPath:@"created_at" toAttribute:@"createdAt"];
[...]
// Categories many-to-many.
[postMapping mapKeyPath:@"categories" toRelationship:@"categories"  withMapping:categoryMapping];

当我运行它时，我得到了错误：'[<__NSCFNumber 0x6c625e0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key id.'

如果有人能给我任何关于如何映射这种关系的线索，我将不胜感激。

Answer 1

出现错误是因为您的 JSON 将每个类别作为主键返回，即：一个 NSNumber。但是您的映射需要一个嵌套对象（将转换为 KVC 对象）。当您的地图尝试访问 KVC 对象（实际上是一个 NSNumber）上的属性“id”时，它会像这样崩溃。

我不确定它是否会起作用，但您可以尝试以下方法：

RKManagedObjectMapping* categoryMapping = //something
categoryMapping.primaryKeyAttribute = @"categoryId";
[categoryMapping mapKeyPath: @"" toAttribute: @"categoryId"];

[..]

[postMapping mapKeyPath: @"categories" toRelationship: @"categories" withMapping: categoryMapping]; 

通常，您会使用 [RKManagedObjectMapping connectRelationship:withObjectForPrimaryKeyAttribute:] 方法，但我不知道这是否适用于多对多关系。您可能必须修改源代码才能执行此操作。要查看的代码在 [RKManagedObjectMappingOperation connectRelationship:] 内。

否则，如果可能的话，我建议您更改 JSON 源，使类别数组如下所示：“categories”：[{“categoryID：1}，{“categoryID”：4}]。如果源不可能要更改，您可以使用委托方法或我在下面给出的 RKObjectLoader 扩展手动修改数据：

.h

typedef void(^RKObjectLoaderWillMapDataBlock)(id* mappableData);

@interface RKObjectLoader (Extended)

@property (nonatomic, copy) RKObjectLoaderWillMapDataBlock onWillMapData;

@end

.m

#import "RKObjectLoader+Extended.h"

#import <objc/runtime.h>

NSString* kOnWillMapDataKey = @"onWillMapData";

@implementation RKObjectLoader (Extended)

- (RKObjectLoaderWillMapDataBlock) onWillMapData {
    return objc_getAssociatedObject(self, &kOnWillMapDataKey);
}

- (void) setOnWillMapData:(RKObjectLoaderWillMapDataBlock) block {
    objc_setAssociatedObject(self, &kOnWillMapDataKey, block, OBJC_ASSOCIATION_COPY);
}

- (RKObjectMappingResult*)mapResponseWithMappingProvider:(RKObjectMappingProvider*)mappingProvider toObject:(id)targetObject inContext:(RKObjectMappingProviderContext)context error:(NSError**)error {
    id<RKParser> parser = [[RKParserRegistry sharedRegistry] parserForMIMEType:self.response.MIMEType];
    NSAssert1(parser, @"Cannot perform object load without a parser for MIME Type '%@'", self.response.MIMEType);

    // Check that there is actually content in the response body for mapping. It is possible to get back a 200 response
    // with the appropriate MIME Type with no content (such as for a successful PUT or DELETE). Make sure we don't generate an error
    // in these cases
    id bodyAsString = [self.response bodyAsString];
    RKLogTrace(@"bodyAsString: %@", bodyAsString);
    if (bodyAsString == nil || [[bodyAsString stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] length] == 0) {
        RKLogDebug(@"Mapping attempted on empty response body...");
        if (self.targetObject) {
            return [RKObjectMappingResult mappingResultWithDictionary:[NSDictionary dictionaryWithObject:self.targetObject forKey:@""]];
        }

        return [RKObjectMappingResult mappingResultWithDictionary:[NSDictionary dictionary]];
    }

    id parsedData = [parser objectFromString:bodyAsString error:error];
    if (parsedData == nil && error) {
        return nil;
    }

    // Allow the delegate to manipulate the data
    if ([self.delegate respondsToSelector:@selector(objectLoader:willMapData:)]) {
        parsedData = AH_AUTORELEASE([parsedData mutableCopy]);
        [(NSObject<RKObjectLoaderDelegate>*)self.delegate objectLoader:self willMapData:&parsedData];
    }

    if( self.onWillMapData ) {
        parsedData = AH_AUTORELEASE([parsedData mutableCopy]);
        self.onWillMapData(&parsedData);
    }

    RKObjectMapper* mapper = [RKObjectMapper mapperWithObject:parsedData mappingProvider:mappingProvider];
    mapper.targetObject = targetObject;
    mapper.delegate = (id<RKObjectMapperDelegate>)self;
    mapper.context = context;
    RKObjectMappingResult* result = [mapper performMapping];

    // Log any mapping errors
    if (mapper.errorCount > 0) {
        RKLogError(@"Encountered errors during mapping: %@", [[mapper.errors valueForKey:@"localizedDescription"] componentsJoinedByString:@", "]);
    }

    // The object mapper will return a nil result if mapping failed
    if (nil == result) {
        // TODO: Construct a composite error that wraps up all the other errors. Should probably make it performMapping:&error when we have this?
        if (error) *error = [mapper.errors lastObject];
        return nil;
    }

    return result;
}

@end