CoreData的外键关联

创建工程时勾选Use Core Data自动生成以下方法

AppDelegate.h

#import 
#import 

@interface AppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;
@property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;
@property (readonly, strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;

//保存数据
- (void)saveContext;
//Documents路径
- (NSURL *)applicationDocumentsDirectory;

@end

AppDelegate.m

//保存上下文,保存数据
- (void)saveContext {
    NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
    if (managedObjectContext != nil) {
        NSError *error = nil;
        if ([managedObjectContext hasChanges] && ![managedObjectContext save:&error]) {
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            abort();
        }
    }
}

//返回Documents的URL路径
- (NSURL *)applicationDocumentsDirectory {
    // The directory the application uses to store the Core Data store file. This code uses a directory named "com.wxhl._2_CoreData______" in the application's documents directory.
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
}

//managedObjectModel属性的GET方法
//自动读取momd文件，创建相对应的数据模型
- (NSManagedObjectModel *)managedObjectModel {

    if (_managedObjectModel != nil) {
        return _managedObjectModel;
    }
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"_2_CoreData______" withExtension:@"momd"];
    _managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    return _managedObjectModel;
}

//psc属性GET方法
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {

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

    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"Day10_02CoreData_____1_N.sqlite"];
    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]) {

        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];
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return _persistentStoreCoordinator;
}

//managedObjectContext属性的GET方法,并且设置了PSC
- (NSManagedObjectContext *)managedObjectContext {

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

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

1对1

    AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;
    //通过实体描述创建第一个对象对象
    Person *person = [NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:appDelegate.managedObjectContext];

    //通过实体描述创建第一个对象对象
    Dog *dog = [NSEntityDescription insertNewObjectForEntityForName:@"Dog" inManagedObjectContext:appDelegate.managedObjectContext];

    //外键关联
    person.dog = dog;
    //保存数据
    [appDelegate saveContext];

1对1数据库实现原理

    /*

     Person持有了Dog
     在数据库文件中：
     (1)ZPERSON表格中添加了ZDOG字段,与ZDOG建立外键关联
     (2)ZPERSON.DOG的值 = ZDOG.Z_PK

     */

1对N

1对N数据库实现原理

    AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;
    //创建多个对象
    Dog *dog1 = [NSEntityDescription insertNewObjectForEntityForName:@"Dog" inManagedObjectContext:appDelegate.managedObjectContext];
    Dog *dog2 = [NSEntityDescription insertNewObjectForEntityForName:@"Dog" inManagedObjectContext:appDelegate.managedObjectContext];
    Dog *dog3 = [NSEntityDescription insertNewObjectForEntityForName:@"Dog" inManagedObjectContext:appDelegate.managedObjectContext];
    //创建狗主人
    Person *person = [NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:appDelegate.managedObjectContext];

    //添加一个外键
    [person addDogsObject:dog1];
    //添加多个外键
    [person addDogs:[NSSet setWithObjects:dog2,dog3, nil]];

1对N实现原理

    /*
     Person持有了多个Dog对象

     Person类中多了如下一个属性
     @property (nullable, nonatomic, retain) NSSet *dogs;

     在数据库文件中：
     (1)ZDOG表格中添加一个字段，Z2DOGS。
     进行外键连接 ZDOG.Z2DOGS ----> ZPERSON.P_PK

     */