Loading Core Data relationships from existing database ID relationships - core-data

Question

I have an application that has lots of data in tables, and I'm writing a mobile app that gets a small portion of the data. The data in tables uses IDs for relations, and I'm trying to load it into a Core Data model and preserve the relationships.
But it doesn't seem like there's an easy way to tell Core Data, "For this relationship, I want a foreign key to this other table." Instead, I have this monstrosity (this is a simplified version of only one of six asynchronous RestKit queries that are coming back to fill the database):
[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]);
}];
I'm using a modified version of Matt Gallagher's One-Line Fetch at http://www.cocoawithlove.com/2008/03/core-data-one-line-fetch.html for the fetchObjectsForEntityName.
Anyway, this seems pretty horrible to me, like I'm missing something obvious. Is there some way to just tell Core Data about database-style foreign keys? Is there an easy way to populate them, if there isn't? Because doing this many fetches for every single entity sure doesn't seem like the right way to do it. And if RestKit helps out here, so much the better.

Answer 1

Nope, but there should be no need to set the inverse relationships.

Answer 2

Thanks to Wain and his insightful comment, I managed to get RestKit to do this for me in a reasonably-elegant fashion. Here is how I set up the foreign key relationships. I set up both ends of each relationship so that either set of data could come in first and the relationship would still get populated properly.
First, I set up the manager, telling it to use JSON serialization:
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;
Next, I set up connections and response descriptors:
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]];
// ...
Then, I was able to do the simple request, and it worked great:
[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]);
}];
Note: This used a few helper methods, which I will reproduce here:
- (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;
}