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:)