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":@11},                           @{
    @"name":@"张惠妹", @"age":@8},                           @{
    @"name":@"井冈山", @"age":@10}];    [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:)