CoreData相关03 表关联

表关联

比如企业下有众多部门，而员工属于某一个部门，则在员工表与部门表之间产生了关联。
在上一篇文章中已经有一张“Person”表了，现在创建一个“国家”表“Country”

添加关联

然后把person类都删掉，重新生成一下person类文件。
重新生成后，就可以发现多出了关联的那个属性了。

添加关联数据

- (void)addPersonAndCountry{
    Person* person = [NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:self.context];
    NSFetchRequest* request = [NSFetchRequest fetchRequestWithEntityName:@"Country"];
    NSPredicate* predicate = nil;
    if (arc4random()%2) {
        predicate = [NSPredicate predicateWithFormat:@"name=%@",@"NaiCha"];
    }else{
        predicate = [NSPredicate predicateWithFormat:@"name=%@",@"KeKoCola"];
    }
    request.predicate = predicate;
    //创建一个数组用于接收查询到的数据对象
    NSError* error = nil;
    NSArray* resultArray = [self.context executeFetchRequest:request error:&error];
    if (error) {
        NSLog(@"查询失败,错误信息:%@",error);
    }else{
        if (resultArray.count == 1) {
            Country* country = resultArray[0];
            //给该模型对象赋值
            int numberID = arc4random()%10000;
            person.name = [NSString stringWithFormat:@"personSomeone%d",numberID];
            int64_t personAge = arc4random()%100;
            person.age = personAge;
            person.country = country;
            [self.context save:&error];
            if (error) {
                NSLog(@"添加个人信息出错，错误:%@",error);
            }else{
                NSLog(@"添加个人信息:%@-%lld-%@",person.name,person.age,person.country.name);
            }
        }else{
            NSLog(@"查询无此国籍，无法添加个人信息");
        }
    }
}

运行结果：

2022-03-04 21:19:48.855904+0800 CoreData实践1[77919:3270501] 添加个人信息:personSomeone6740-84-KeKoCola

不用写任何数据库 *** 作的代码，直接将关联的对象当作属性成员赋值就完成关联了，非常方便。

查找关联数据

- (void)findSomebodyByCountry:(NSString*)countryName{
    //查找国籍为“奶茶”的人
    NSFetchRequest* request = [NSFetchRequest fetchRequestWithEntityName:@"Person"];
    NSPredicate* predicate = [NSPredicate predicateWithFormat:@"country.name=%@",countryName];
    request.predicate = predicate;
    NSError* error = nil;
    NSArray* resultArray = [self.context executeFetchRequest:request error:&error];
    if (error) {
        NSLog(@"查询失败,错误信息:%@",error);
    }else{
        NSLog(@"查询国籍为:%@的结果如下",countryName);
        for (Person* person in resultArray) {
            NSLog(@"name:%@ age:%lld country:%@",person.name,person.age,person.country.name);
        }
    }
}

运行结果:

2022-03-04 23:17:51.282876+0800 CoreData实践1[78883:3309210] 查询国籍为:NaiCha的结果如下
2022-03-04 23:17:51.283777+0800 CoreData实践1[78883:3309210] name:personSomeone8102 age:43 country:NaiCha
2022-03-04 23:17:51.283962+0800 CoreData实践1[78883:3309210] name:personSomeone5777 age:17 country:NaiCha
2022-03-04 23:17:51.284096+0800 CoreData实践1[78883:3309210] name:personSomeone966 age:17 country:NaiCha
2022-03-04 23:17:51.284221+0800 CoreData实践1[78883:3309210] name:personSomeone7477 age:57 country:NaiCha
2022-03-04 23:17:51.284329+0800 CoreData实践1[78883:3309210] name:personSomeone701 age:12 country:NaiCha
2022-03-04 23:17:51.284435+0800 CoreData实践1[78883:3309210] name:personSomeone2071 age:47 country:NaiCha
2022-03-04 23:17:51.284553+0800 CoreData实践1[78883:3309210] name:personSomeone1474 age:78 country:NaiCha

直接使用点语法增加书写约束语句即可实现关联数据的查询。