基于返回NULL的关系的Core Data属性

Question

我有一个核心数据数据模型，例如

我想在UITableViewCell中显示与每个Batting对象关联的Teams.name。当我尝试访问batting.teams.name时，它返回NULL。

我的开放视图控制器传入了一个字符串，然后根据Master.nameLast创建一个带有NSPredicate的新viewController，并列出所有匹配的名称

和NSFetchedResultsController创建了上面的列表

#pragma mark - Fetched Results Controller Section

- (NSFetchedResultsController *)fetchedResultsController {

    if(_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSManagedObjectContext *context = [self managedObjectContext];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Master" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"debut" ascending:YES];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"nameLast = [c]%@", lastName];
    [fetchRequest setPredicate:predicate];

    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
    fetchRequest.sortDescriptors = sortDescriptors;

    _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:context sectionNameKeyPath:nil cacheName:nil];

    return _fetchedResultsController;
}

我的下一个视图控制器然后列出与所选主NSManagedObject关联的所有Batting属性。

但是当我尝试访问batting.teams.name时，它返回NULL !!!!!如果我从我的代表NSLog实体，他们已经设置但我无法从它的Batting.teams关系中访问Teams.name

#pragma mark - Fetched Results Controller Section

- (NSFetchedResultsController *)fetchedResultsController {
    if(_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSManagedObjectContext *context = [self managedObjectContext];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Batting" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"yearID" ascending:YES];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"playerID = %@ " , selectedMaster.playerID];
    [fetchRequest setPredicate:predicate];

    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
    fetchRequest.sortDescriptors = sortDescriptors;

    _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:context sectionNameKeyPath:nil cacheName:nil];

    return _fetchedResultsController;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"playerStats" forIndexPath:indexPath];

    Batting *batting = [self.fetchedResultsController objectAtIndexPath:indexPath];
    NSString *team = batting.teams.name;  //NULL NULL NULL



    // Configure the cell...
    double average = [batting.h doubleValue]/[batting.ab doubleValue];
    NSString *averageStr = [[NSString stringWithFormat:@"%.3f", average] substringFromIndex:1];

    cell.textLabel.text = [NSString stringWithFormat:@"%@ %@", batting.yearID, batting.teams.name];
    cell.detailTextLabel.text = [NSString stringWithFormat:@"H: %@, AVG: %@, HR: %@, RBI: %@, SB: %@, R: %@, BB: %@, K: %@",
                                 batting.h, averageStr , batting.hr, batting.rbi, batting.sb, batting.r, batting.bb, batting.so];
    cell.detailTextLabel.textColor = [UIColor grayColor];

    return cell;
}

为第二个参数????????

返回null

cell.textLabel.text = [NSString stringWithFormat:@"%@ %@", batting.yearID, batting.teams.name];

这是从AppDelegate登录的团队实体

//
//  AppDelegate.m
//  Baseball Stats
//
//  Created by Jason Steindorf on 6/6/15.
//  Copyright (c) 2015 Jason Steindorf. All rights reserved.
//

#import "AppDelegate.h"
#import "Master.h"
#import "Teams.h"
#import "Batting.h"


@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.

    NSError *error;

    NSString* dataPath_MASTER = [[NSBundle mainBundle] pathForResource:@"master" ofType:@"json"];
    NSArray* MASTER = [NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:dataPath_MASTER]
                                                      options:kNilOptions
                                                        error:&error];

    NSString* dataPath_TEAMS = [[NSBundle mainBundle] pathForResource:@"teams" ofType:@"json"];
    NSArray* TEAMS = [NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:dataPath_TEAMS]
                                                      options:kNilOptions
                                                        error:&error];

    NSString* dataPath_ABC = [[NSBundle mainBundle] pathForResource:@"abc" ofType:@"json"];
    NSArray* BATTING_ABC = [NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:dataPath_ABC]
                                                       options:kNilOptions
                                                         error:&error];

    NSString* dataPath_DEFGH = [[NSBundle mainBundle] pathForResource:@"defgh" ofType:@"json"];
    NSArray* BATTING_DEFGH = [NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:dataPath_DEFGH]
                                                       options:kNilOptions
                                                         error:&error];

    NSString* dataPath_IJKLM = [[NSBundle mainBundle] pathForResource:@"ijklm" ofType:@"json"];
    NSArray* BATTING_IJKLM = [NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:dataPath_IJKLM]
                                                       options:kNilOptions
                                                         error:&error];

    NSString* dataPath_NOPQR = [[NSBundle mainBundle] pathForResource:@"nopqr" ofType:@"json"];
    NSArray* BATTING_NOPQR = [NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:dataPath_NOPQR]
                                                       options:kNilOptions
                                                         error:&error];

    NSString* dataPath_ST = [[NSBundle mainBundle] pathForResource:@"st" ofType:@"json"];
    NSArray* BATTING_ST = [NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:dataPath_ST]
                                                      options:kNilOptions
                                                        error:&error];

    NSString* dataPath_UVWXYZ = [[NSBundle mainBundle] pathForResource:@"uvwxyz" ofType:@"json"];
    NSArray* BATTING_UVWXYZ = [NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:dataPath_UVWXYZ]
                                                          options:kNilOptions
                                                            error:&error];

    // Test listing all Master from the store
    NSFetchRequest *fetchRequestMaster = [[NSFetchRequest alloc] init];
    NSEntityDescription *entityMaster = [NSEntityDescription entityForName:@"Master" inManagedObjectContext:self.managedObjectContext];
    [fetchRequestMaster setEntity:entityMaster];

    NSArray *fetchedObjectsMaster = [self.managedObjectContext executeFetchRequest:fetchRequestMaster error:&error];
    NSLog(@"Number of records in master.json - %d", (int)[fetchedObjectsMaster count]);

    for (id m in MASTER) {
        Master *master = [NSEntityDescription insertNewObjectForEntityForName:@"Master"
                                                       inManagedObjectContext:self.managedObjectContext];

        master.nameFirst = [m objectForKey:@"nameFirst"];
        master.nameLast  = [m objectForKey:@"nameLast"];
        master.debut     = [m objectForKey:@"debut"];
        master.playerID  = [m objectForKey:@"playerID"];
        NSError *error;
        if (![self.managedObjectContext save:&error]) {
            NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
        }
    }
    fetchedObjectsMaster = [self.managedObjectContext executeFetchRequest:fetchRequestMaster error:&error];

    for (Master *master in fetchedObjectsMaster) {
        NSLog(@"first name: %@", master.nameFirst);
        NSLog(@"last name:  %@", master.nameLast);
        NSLog(@"debut:      %@", master.debut);
        NSLog(@"playerID:   %@\n\n", master.playerID);
    }


    // Test listing all Master from the store
    NSFetchRequest *fetchRequestTeams = [[NSFetchRequest alloc] init];
    NSEntityDescription *entityTeams = [NSEntityDescription entityForName:@"Teams" inManagedObjectContext:self.managedObjectContext];
    [fetchRequestTeams setEntity:entityTeams];

    NSArray *fetchedObjectsTeams = [self.managedObjectContext executeFetchRequest:fetchRequestTeams error:&error];
    NSLog(@"Number of records in teams.json - %d", (int)[fetchedObjectsTeams count]);

    for (id t in TEAMS) {
        Teams *team = [NSEntityDescription insertNewObjectForEntityForName:@"Teams"
                                                    inManagedObjectContext:self.managedObjectContext];

        team.name       = [t objectForKey:@"name"];
        team.minYearID  = [t objectForKey:@"minYearID"];
        team.maxYearID  = [t objectForKey:@"maxYearID"];
        team.teamID     = [t objectForKey:@"teamID"];

        NSError *error;
        if (![self.managedObjectContext save:&error]) {
            NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
        }
    }
    fetchedObjectsTeams = [self.managedObjectContext executeFetchRequest:fetchRequestTeams error:&error];

    for (Teams *team in fetchedObjectsTeams) {
        NSLog(@"name:       %@", team.name);
        NSLog(@"minYearID:  %@", team.minYearID);
        NSLog(@"maxYearID:  %@", team.maxYearID);
        NSLog(@"teamID:     %@\n\n", team.teamID);
    }


    NSFetchRequest *fetchRequestBatting = [[NSFetchRequest alloc] init];
    NSEntityDescription *entityBatting = [NSEntityDescription entityForName:@"Batting" inManagedObjectContext:self.managedObjectContext];
    [fetchRequestBatting setEntity:entityBatting];

    NSArray *fetchedObjectsBatting = [self.managedObjectContext executeFetchRequest:fetchRequestBatting error:&error];
    NSLog(@"Number of records in battingFiles.json - %d", (int)[fetchedObjectsBatting count]);

    for (id b in BATTING_UVWXYZ) {
        Batting *batting = [NSEntityDescription insertNewObjectForEntityForName:@"Batting"
                                                         inManagedObjectContext:self.managedObjectContext];

        batting.playerID    = [b objectForKey:@"playerID"];
        batting.h           = [b objectForKey:@"h"];
        batting.ab          = [b objectForKey:@"ab"];
        batting.hr          = [b objectForKey:@"hr"];
        batting.rbi         = [b objectForKey:@"rbi"];
        batting.sb          = [b objectForKey:@"sb"];
        batting.r           = [b objectForKey:@"r"];
        batting.bb          = [b objectForKey:@"bb"];
        batting.so          = [b objectForKey:@"so"];
        batting.yearID      = [b objectForKey:@"yearID"];
        batting.teamID      = [b objectForKey:@"teamID"];

        NSError *error;
        if (![self.managedObjectContext save:&error]) {
            NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
        }
    }

    fetchedObjectsBatting = [self.managedObjectContext executeFetchRequest:fetchRequestBatting error:&error];

    for (Batting *b in fetchedObjectsBatting) {
        NSLog(@"playerID:   %@", b.playerID);
        NSLog(@"h:          %@", b.h);
        NSLog(@"ab:         %@", b.ab);
        NSLog(@"hr:         %@", b.hr);
        NSLog(@"rbi:        %@", b.rbi);
        NSLog(@"sb:         %@", b.sb);
        NSLog(@"r:          %@", b.r);
        NSLog(@"bb:         %@", b.bb);
        NSLog(@"so:         %@", b.so);
        NSLog(@"yearID:     %@", b.yearID);
        NSLog(@"teamID:     %@\n\n", b.teamID);
    }

    return YES;
}

- (void)applicationWillResignActive:(UIApplication *)application {
    // Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
    // Use this method to pause ongoing tasks, disable timers, and throttle down OpenGL ES frame rates. Games should use this method to pause the game.
}

- (void)applicationDidEnterBackground:(UIApplication *)application {
    // Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
    // If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
}

- (void)applicationWillEnterForeground:(UIApplication *)application {
    // Called as part of the transition from the background to the inactive state; here you can undo many of the changes made on entering the background.
}

- (void)applicationDidBecomeActive:(UIApplication *)application {
    // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
}

- (void)applicationWillTerminate:(UIApplication *)application {
    // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
    // Saves changes in the application's managed object context before the application terminates.
    [self saveContext];
}

#pragma mark - Core Data stack

@synthesize managedObjectContext = _managedObjectContext;
@synthesize managedObjectModel = _managedObjectModel;
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;

- (NSURL *)applicationDocumentsDirectory {
    // The directory the application uses to store the Core Data store file. This code uses a directory named "steindorf.Baseball_Stats" in the application's documents directory.
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
}

- (NSManagedObjectModel *)managedObjectModel {
    // The managed object model for the application. It is a fatal error for the application not to be able to find and load its model.
    if (_managedObjectModel != nil) {
        return _managedObjectModel;
    }
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"Baseball_Stats" withExtension:@"momd"];
    _managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    return _managedObjectModel;
}

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
    // The persistent store coordinator for the application. This implementation creates and return a coordinator, having added the store for the application to it.
    if (_persistentStoreCoordinator != nil) {
        return _persistentStoreCoordinator;
    }

    // Create the coordinator and store

    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"Baseball_Stats.sqlite"];
    NSLog(@"%@", storeURL);


    NSError *error = nil;
    NSString *failureReason = @"There was an error creating or loading the application's saved data.";
    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {
        // Report any error we got.
        NSMutableDictionary *dict = [NSMutableDictionary dictionary];
        dict[NSLocalizedDescriptionKey] = @"Failed to initialize the application's saved data";
        dict[NSLocalizedFailureReasonErrorKey] = failureReason;
        dict[NSUnderlyingErrorKey] = error;
        error = [NSError errorWithDomain:@"YOUR_ERROR_DOMAIN" code:9999 userInfo:dict];
        // Replace this with code to handle the error appropriately.
        // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return _persistentStoreCoordinator;
}


- (NSManagedObjectContext *)managedObjectContext {
    // Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.)
    if (_managedObjectContext != nil) {
        return _managedObjectContext;
    }

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (!coordinator) {
        return nil;
    }
    _managedObjectContext = [[NSManagedObjectContext alloc] init];
    [_managedObjectContext setPersistentStoreCoordinator:coordinator];
    return _managedObjectContext;
}

#pragma mark - Core Data Saving support

- (void)saveContext {
    NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
    if (managedObjectContext != nil) {
        NSError *error = nil;
        if ([managedObjectContext hasChanges] && ![managedObjectContext save:&error]) {
            // Replace this implementation with code to handle the error appropriately.
            // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            abort();
        }
    }
}

@end

Answer 1

In the block below, you need to assign the team to batting.

batting.teams = {your_team_object}

Then you will be able to access the team information. You can create another fetch to Core Data using the teamID to retrieve the team object.

for (id b in BATTING_UVWXYZ) {
    Batting *batting = [NSEntityDescription insertNewObjectForEntityForName:@"Batting"
                                                     inManagedObjectContext:self.managedObjectContext];

    batting.playerID    = [b objectForKey:@"playerID"];
    batting.h           = [b objectForKey:@"h"];
    batting.ab          = [b objectForKey:@"ab"];
    batting.hr          = [b objectForKey:@"hr"];
    batting.rbi         = [b objectForKey:@"rbi"];
    batting.sb          = [b objectForKey:@"sb"];
    batting.r           = [b objectForKey:@"r"];
    batting.bb          = [b objectForKey:@"bb"];
    batting.so          = [b objectForKey:@"so"];
    batting.yearID      = [b objectForKey:@"yearID"];
    batting.teamID      = [b objectForKey:@"teamID"];

NSPredicate *teamIDPredicate = [NSPredicate predicateWithFormat:@"teamID == %@", [b objectForKey:@"teamID"]];
[fetchRequestTeams setPredicate:predicate];
NSArray *fetchedSpecificTeam = [self.managedObjectContext executeFetchRequest:fetchRequestTeams error:&error];

batting.teams= [fetchedSpecificTeam  objectAtIndex:0];


    NSError *error;
    if (![self.managedObjectContext save:&error]) {
        NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
    }
}