核心数据如何确保实体中只有一个对象？

Question

我有一个用户表和一个女孩表和一个girls_result表.

基本上我只希望USER实体中有一个用户,USER是设备所有者,我想在每次需要时调用同一个USER.

USER可以有多个女孩,女孩可以有多个结果.
数据模型: USER <------>>GIRLS<-------->>GIRLS_RESULTS

所以我将user_deviceid属性添加到USER Entity并创建了一个单例类,这样我就可以为用户设置一个唯一的编号,并在需要时调用它.我想确保没有创建具有相同UUID的多个用户对象.

@implementation SingletonClass

@synthesize singletonManagedObjectContext=_singletonManagedObjectContext;
@synthesize userIS=_userIS;

+ (SingletonClass *)sharedInstance
{
    static SingletonClass *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[SingletonClass alloc] init];
        // Do any other initialisation stuff here
    });
    return sharedInstance;
}

- (id)init {
    if (self = [super init]) {

        // set a singleton managed object context
        _singletonManagedObjectContext=self.singletonManagedObjectContext;

        //set only one user based on unique device id, so all tables will belong to that user
        NSManagedObjectContext *context = _singletonManagedObjectContext;

        //how to make sure that _userIS is the same user all the time???

        if (_userIS.user_deviceid == [self newUUID]) {
            NSLog(@"user dvice id matches");
        }
        else{
            _userIS.user_deviceid=[self newUUID];
        }


    }
    return self;
}

- (NSString *)newUUID
{
    CFUUIDRef theUUID = CFUUIDCreate(NULL);
    CFStringRef string = CFUUIDCreateString(NULL, theUUID);
    CFRelease(theUUID);
    return (NSString *)CFBridgingRelease(string);
} 

那么如何确保在USER实体中只创建一个USER,并且每次都可以使用该特定的USER？

============编辑答案============基于接受的答案下面我创建了一个它工作正常的课程

#import "CheckUser.h"
#import "USER.h"
#import "SingletonClass.h"

@implementation CheckUser


- (USER *)checkandreturnUser
{
    SingletonClass *sharedInstance = [SingletonClass sharedInstance];
    NSManagedObjectContext *context = sharedInstance.singletonManagedObjectContext;
    // Fetch Form Object
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    [fetchRequest setEntity:[NSEntityDescription entityForName:@"USER"
                                        inManagedObjectContext:context]];
    USER *user;
    NSError *error = nil;
    NSArray *userArray = [context executeFetchRequest:fetchRequest error:&error];
    NSLog(@"user array count %i",[userArray count]);
    if (userArray == nil) {
        NSAssert(0, @"There was an error fetching user object");
        userArray = [NSArray array];
    }
    // If user doesn't exist create it
    if ([userArray count] > 0) {
        NSAssert([userArray count] == 1, @"Expected one user but there were more");
        user = [userArray objectAtIndex:0];
    } else {
        //Create user object
        user = [NSEntityDescription insertNewObjectForEntityForName:@"USER"
                                             inManagedObjectContext:context];
        user.user_deviceid=[self newUUID];
        // maybe save here?
        if (![context save:&error]) {
            NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
        }
    }


    return user;
}
- (NSString *)newUUID
{
    CFUUIDRef theUUID = CFUUIDCreate(NULL);
    CFStringRef string = CFUUIDCreateString(NULL, theUUID);
    CFRelease(theUUID);
    return (NSString *)CFBridgingRelease(string);
}

Answer 1

核心数据不支持单身实体的想法.如果您想在数据存储中只有一个实体实例,则需要编写应用程序以确保永远不会创建多个实体.

原因很简单,这是一个奇怪的命题,不符合数据存储的想法.如果您只有一个实例,则不需要为该实例使用Core Data.将单例数据保存在用户默认值或自定义文件中.将它放在数据存储中并没有多大意义.您不需要它与其他对象之间的关系,因为您总是知道这些关系是什么 - 它们是不可配置的,它们不需要被查找,因此它们不需要存在.