RestKit: connecting up a relationship which isn't in the response - objective-c

Question

Suppose that in my data model, I have Pages, which have-many Comments.
I want to connect up a relationship in the model which goes from the Comment back to the Page it belongs to, but the Page object isn't nested in the response, nor is any primary key which could identify the parent Page present in the response.
At the time that I call loadObjectsAtResourcePath, all the Comments which are loaded should belong to a fixed, known Page object. One way I could hook up the relationship would be to do:
loader.onDidLoadObjects = ^(NSArray* objs) {
for (Comment* comment in objs) comment.page = self.page;
...
}
but I'm hoping there's a better way. Note that I can't use the connectRelationship family of methods, because there's no primary key in the response which could let me hook each Comment up to a Page.

Answer 1

You can use the delegate method - (void)objectLoader:(RKObjectLoader *)loader willMapData:(inout id *)mappableData to inject extra parameters before the mapping step. It ensures the objects and relationships will be correctly saved by RestKit if you are using core data (note the solution you gave above does not save the relationship).
Alternatively, look at this answer where I showed how to override RKObjectLoader to retrieve the page information from the URL itself.
EDIT: Here is the category I mentioned in the comment:
.h
#import <RestKit/RestKit.h>
typedef void(^RKObjectLoaderWillMapDataBlock)(id* mappableData);
#interface RKObjectLoader (Extended)
#property (nonatomic, copy) RKObjectLoaderWillMapDataBlock onWillMapData;
#end
and the .m:
#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 = [parsedData mutableCopy];
[(NSObject<RKObjectLoaderDelegate>*)self.delegate objectLoader:self willMapData:&parsedData];
}
if( self.onWillMapData ) {
parsedData = [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