RestKit多对多映射,如何？

Question

我正在研究我的第一个RestKit应用程序,并使用CoreData进行本地缓存.在其中我有Post << --- >> Category之间的多对多关系.这种关系称为帖子和类别.

在我的Post JSON调用中,我获得了类似这样的类别{"post": [...] "categories":[1,4] [...]的id:,其中1和4是id.

我有一个Post继承自定义的模型对象NSManagedObject,在其中我有类别的属性,如下所示:

@interface Post : NSManagedObject

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

我在自定义的Category模型对象中做同样的事情.

我的映射目前看起来像这样:

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];

当我运行这个时,我得到错误: '[<__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,我将非常感激.

Answer 1

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

我不确定它是否有效，但你可以尝试这样的事情：

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。您可能需要修改源代码才能执行此操作。要查看的代码位于 [RKManagedObjectMappingOperation connectRelationship:] 内部。

否则，如果可能的话，我建议您更改 JSON 源，以便类别数组如下所示： "categories" : [{"categoryID: 1}, {"categoryID" : 4}]。如果无法更改源，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

.米

#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