CoreData - 向现有实体添加新关系答案

【问题标题】：CoreData - Adding a new relationship to existing entityCoreData - 向现有实体添加新关系
【发布时间】：2011-02-15 15:04:03
【问题描述】：

我有以下基本问题：

我有两个实体，个人和部门。

在添加新人之前，我要检查该部门是否已存在，如果存在，则将新人链接到现有部门。

带有新部门关系的简单插入：

    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。

但是如果我想在上面的代码执行后做以下事情：

    [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，我的问题是表格部门中的重复条目。

也许有人知道一个很好的 CoreData 教程，它适合具有高级 SQL 知识的初学者。（在谷歌上搜索或阅读开发者文档几个小时并没有我想的那么有用：））

对于作为初学者的我来说，重要的是现在我是否走在正确的道路上，有人可以确认这一点吗？

谢谢你，

马蒂亚斯

【问题讨论】：

标签： iphone core-data relationship

【解决方案1】：

您在if/else 语句中定义了部门变量，然后在它之外使用了另一个变量。尝试将您的 if 更改为：

[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，但我们帮不了你太多。 . .

【讨论】：

【解决方案2】：

对不起，检查功能是这样的：

- (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];
}

【讨论】：