使用CoreData [2]

使用CoreData [2]

此篇讲解CoreData处理关系型数据.

1. 先创建出Student于Teacher的实体.

2. 确定关系,并修改描述

3. 创建对象,并查看一下关系(Teacher与Student之间是有着关联关系的哦)

4. 测试代码.

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    NSLog(@"%@", NSHomeDirectory());

    // 创建老师的实体
    Teacher *KongZi = [self createTeacher:@"KongZi"];

    // 创建学生的实体并添加到老师的实体当中
    NSArray *students =  @[@{@"name":@"王力宏", @"age":@},
                           @{@"name":@"张惠妹", @"age":@},
                           @{@"name":@"井冈山", @"age":@}];
    [students enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        Student *student = [self createStudent:obj[@"name"] age:obj[@"age"]];

        [KongZi addStudentsObject:student];
    }];

    // 存储信息
    [self saveContext];

    return YES;
}

- (Student *)createStudent:(NSString *)name age:(NSNumber *)age
{
    // 实体描述信息
    NSEntityDescription *description = \
    [NSEntityDescription entityForName:@"Student"
                inManagedObjectContext:[self managedObjectContext]];

    // 初始化对象
    Student *student = [[Student alloc] initWithEntity:description
                        insertIntoManagedObjectContext:[self managedObjectContext]];
    student.name     = name;
    student.age      = age;

    return student;
}

- (Teacher *)createTeacher:(NSString *)name
{
    // 实体描述信息
    NSEntityDescription *description = \
    [NSEntityDescription entityForName:@"Teacher"
                inManagedObjectContext:[self managedObjectContext]];

    // 初始化对象
    Teacher *teacher = [[Teacher alloc] initWithEntity:description
                        insertIntoManagedObjectContext:[self managedObjectContext]];
    teacher.name     = name;

    return teacher;
}

5. 验证关系型数据

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    NSLog(@"%@", NSHomeDirectory());

    // 设定要查询的实体
    NSFetchRequest *fetch = [NSFetchRequest fetchRequestWithEntityName:@"Teacher"];

    // 取出查询结果
    NSArray *teachers = [[self managedObjectContext] executeFetchRequest:fetch error:nil];

    // 遍历出所有老师
    [teachers enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        Teacher *teacher = obj;

        // 获取该老师所有的学生
        [[teacher students] enumerateObjectsUsingBlock:^(id obj, BOOL *stop) {
            Student *student = obj;
            NSLog(@"%@ %@", student.age, student.name);

            Teacher *tmp = (Teacher *)student.teacher;
            NSLog(@"老师是: %@", tmp.name);
        }];
    }];

    return YES;
}

*6. 验证存在两个老师的情形

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // 本人意淫一回当老师
    Teacher *YouXianMing = [self createTeacher:@"游贤明"];

    // 设定要查询的实体
    NSFetchRequest *fetch = [NSFetchRequest fetchRequestWithEntityName:@"Teacher"];

    // 取出查询结果
    NSArray *teachers = [[self managedObjectContext] executeFetchRequest:fetch error:nil];

    // 遍历出所有老师
    [teachers enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        Teacher *teacher = obj;

        // 获取该老师所有的学生
        [[teacher students] enumerateObjectsUsingBlock:^(id obj, BOOL *stop) {
            Student *student = obj;

            // 添加一个学生后就停止
            [YouXianMing addStudentsObject:student];
            *stop = YES;
        }];
    }];

    // 存储信息
    [self saveContext];

    return YES;
}

执行上述代码之后在执行下述代码.

下图表明了这个原因:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // 设定要查询的实体
    NSFetchRequest *fetch = [NSFetchRequest fetchRequestWithEntityName:@"Teacher"];

    // 取出查询结果
    NSArray *teachers = [[self managedObjectContext] executeFetchRequest:fetch error:nil];

    // 遍历出所有老师
    [teachers enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        Teacher *teacher = obj;

        // 获取该老师所有的学生
        [[teacher students] enumerateObjectsUsingBlock:^(id obj, BOOL *stop) {
            Student *student = obj;
            Teacher *tmp = (Teacher *)student.teacher;

            NSLog(@"%@ %@", student.age, student.name);
            NSLog(@"老师是: %@", tmp.name);
        }];
    }];

    return YES;
}

附录:

如果想修改多对多,请以下图为参考

上图那么修改了之后就会变成下图所示的多对多了,一个学生也可以有好几个老师了.

以上就讲完了关系型数据:)