CoreData表关联详解

在企业中，通常会有多个部门，每个员工隶属于某个部门。这种情况下，员工表和部门表之间就会形成关联关系。例如，在上一篇文章中我们已经创建了一张“Person”表，现在我们将创建一个“Country”表来演示表关联。

首先，我们需要在CoreData模型中添加关联关系。假设我们已经有一个“Person”类，现在需要添加一个“Country”类，并建立两者之间的关联关系。

接下来，删除原有的“Person”类文件，重新生成新的“Person”类文件。重新生成后，你会发现“Person”类中多了一个关联属性。

- (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;
    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];
            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

通过上述代码，我们可以看到，不需要编写任何复杂的数据库操作代码，直接将关联的对象作为属性成员赋值即可完成关联，非常方便。

- (void)findSomebodyByCountry:(NSString *)countryName {
    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

通过点语法增加约束条件，即可轻松实现关联数据的查询。