CoreData - 向现有实体添加新关系

Question

我有以下基本问题：

我有两个实体，人员和部门。

在添加新人员之前，我想检查该部门是否尚不存在，如果存在，则将新人员链接到现有部门。

简单插入新的部门关系：

    Department *department = (Department *)[NSEntityDescription insertNewObjectForEntityForName:@"Department" inManagedObjectContext:self.context];
    department.groupName = @"Office of Personnel Management";

    Person *person1 = (Person *)[NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:self.context];
    person1.name = @"John Smith";
    person1.birthday = [self dateFromString:@"12-1-1901"];
    person1.department = department;

    Person *person2 = (Person *)[NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:self.context];
    person2.name = @"Jane Doe";
    person2.birthday = [self dateFromString:@"4-13-1922"];
    person2.department = department;

    department.manager = person1;
    department.members = [NSSet setWithObjects:person1, person2, nil];

最后一行建立链接 - 没关系。

但是，如果我想在执行上面的代码后执行以下操作，该怎么办：

    [self checkForExistingDepartment:@"Office of Personnel Management"];
    if(self.existingDepartment) {

        // here is my Problem number 1:
        // department = ???
        (NSEntityDescription *) department = self.existingDepartment;

    } else {
        Department *department = (Department *)[NSEntityDescription insertNewObjectForEntityForName:@"Department" inManagedObjectContext:self.context];
        department.groupName = @"Office of Personnel Management";

    }   

    Person *person1 = (Person *)[NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:self.context];
    person1.name = @"John Smith the second one";
    person1.birthday = [self dateFromString:@"12-1-1901"];
    person1.department = department;

    // former line for adding new: department.members = [NSSet setWithObjects:person1, nil];

    // and here is my problem number 2:
    // I think I need something like:  [NSSet addObjects:person1, nil];

简而言之，我的问题是表部门中的重复条目。

也许有人知道一个很好的 CoreData 教程，这对于具有高级 SQL 知识的初学者很有帮助。 （在谷歌上搜索或阅读开发人员文档几个小时并没有我想象的那么有帮助:)）

对于我作为初学者来说，现在重要的是我是否走在正确的道路上，有人可以证实这一点吗？

谢谢和问候，

马蒂亚斯

Answer 1

您在 if/else 语句内定义了 Department 变量，然后在其外部使用不同的变量。尝试将您的 if 更改为如下所示：

[self checkForExistingDepartment:@"Office of Personnel Management"];
if(self.existingDepartment) {
    department = self.existingDepartment;
} else {
    department = (Department *)[NSEntityDescription insertNewObjectForEntityForName:@"Department" inManagedObjectContext:self.context];
    department.groupName = @"Office of Personnel Management";
}

虽然没有看到 checkForExistingDepartment 的代码，但我们无法为您提供太多帮助。 。 。

Answer 2

抱歉，检查功能如下所示：

- (void) checkForExistingDepartment:(NSString *)searchDepartment {
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Department" initManagedObjectContext:self.context];
    [fetchRequest setEntity:entity];
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"country" ascending:YES selector:nil];
    NSArray *descriptors = [NSArray arrayWithObject:sortDescriptor];
    [fetchRequest setSortDescriptor:descriptors];

    NSString *query = searchDepartment;
    if(query && query.length) {
        fetchRequest.predicate = [NSPredicate predicatWithFormat:@"country contains[cd] %@", query];
    }

    NSError *error;
    self.fetchedResultsController = [[NSFetchedResultsController alloc]
                                     initWithFetchRequest:fetchRequest
                                     managedObjectContext:self.context
                                       sectionNameKeyPath:@"section"
                                                cacheName:@"Root"];
    self.fetchedResultsController.delegate = self;
    [self.fetchedResultsController release];
    if(![[self fetchedResultsController] performFetch:&error]) {
        NSLog(@"Error %@", [error localizedDescription]);
    }

    self.existingDepartment = [self.fetchedResultsController objectAtIndexPath:0];
    [fetchRequest release];
    [sortDescriptor release];
}