一对多关系：CoreData [英] One-to-Many Relationship: CoreData

问题描述

我对coredata相当陌生，并且一直困扰于一个问题。
任何帮助将不胜感激。

I'm fairly new to coredata and have been stuck on an issue. Any assistance will be greatly appreciated.

信息：
我有一个用于实体的核心数据应用程序；
列表和任务。
列表和任务具有一对多关系。

Information: I have a core data app with to entities; List and Task. List and Task have a one-to-many relationship.

Task.h

@class List;

@interface Task : NSManagedObject

@property (nonatomic, retain) NSString * task;
@property (nonatomic, retain) NSString * note;
@property (nonatomic, retain) List *list;

@end

List.h

@class Task;

@interface List : NSManagedObject

@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSDate * dateCreated;
@property (nonatomic, retain) NSNumber * sortOrder;
@property (nonatomic, retain) NSSet *task;
@end

@interface List (CoreDataGeneratedAccessors)

- (void)addTaskObject:(Task *)value;
- (void)removeTaskObject:(Task *)value;
- (void)addTask:(NSSet *)values;
- (void)removeTask:(NSSet *)values;

@end

我使用创建列表；

 NSManagedObjectContext *context = [self managedObjectContext];

if ([self.listTextField.text length] == 0) { // Quit here if no text is entered
    [self dismissViewControllerAnimated:YES completion:nil];
    return;
}

// Create a new list.
// Create an NSManagedObject for our database entity.
list = [NSEntityDescription insertNewObjectForEntityForName:@"List" inManagedObjectContext:context];
// Add the new task to the object (which in turns adds to our database).
[list setValue:self.listTextField.text forKey:@"name"];

// Get current date and time.
NSDate *todayDate = [NSDate date];
// Add the date to the object (which in turns adds to our database).
[list setValue:todayDate forKey:@"dateCreated"];


NSError *error = nil;
// Save the object to persistent store
if (![context save:&error]) {
    NSLog(@"Can't Save! %@ %@", error, [error localizedDescription]);
}
[self dismissViewControllerAnimated:YES completion:nil];

我可以使用下面的代码通过cellaccessorybuttontapped来更新已创建的列表；

I can update already created lists using the code below with cellaccessorybuttontapped;

NSManagedObjectContext *context = [self managedObjectContext];

if ([self.listTextField.text length] == 0) {
    // Delete object from database.
    [context deleteObject:self.list];
    NSError *error = nil;
    // Save the action to persistent store
    if (![context save:&error]) {
        NSLog(@"Can't Save! %@ %@", error, [error localizedDescription]);
    }
    [self dismissViewControllerAnimated:YES completion:nil];
} else {
    // Update existing task.
    [self.list setValue:self.listTextField.text forKey:@"name"];
    // Get current date and time.
    NSDate *todayDate = [NSDate date];
    // Add the date to the object (which in turns adds to our database).
    [list setValue:todayDate forKey:@"dateCreated"];
    NSError *error = nil;
    // Save the action to persistent store
    if (![context save:&error]) {
        NSLog(@"Can't Save! %@ %@", error, [error localizedDescription]);
    }
}
[self dismissViewControllerAnimated:YES completion:nil];

然后我可以导航至列表。

I can then navigate into a list.

我的问题是我该如何为刚刚导航到的列表创建一个任务？
已经有2天了，我一直没能在Google上找到任何东西。

My question is how can I then create a task for the list I have just navigated into? It's been 2 days and I've not been able to find anything on Google.

我建议已添加；

@property (strong, nonatomic) List *selectedList;

我现在将其作为保存方法

I now have this as my Save method

NSManagedObjectContext * context = [selfmanagedObjectContext];

NSManagedObjectContext *context = [self managedObjectContext];

// Saving a new task.
Task *task = [NSEntityDescription insertNewObjectForEntityForName:@"Task" inManagedObjectContext:context];

task.task = self.taskText.text;
task.note = self.noteText.text;
task.list = self.selectedList;
NSLog(@"The selected list is: %@", [self.selectedList description]);

NSError *error = nil;
// Save the object to persistent store
if (![context save:&error]) {
    NSLog(@"Can't Save! %@ %@", error, [error localizedDescription]);
}
[self.selectedList addTaskObject:task];
[self dismissViewControllerAnimated:YES completion:nil];

新任务已创建，但在所有列表中均已创建。
是否可能正在运行，而我不是根据任务列表来获取任务？

The new task is created but it is created in all lists. Is it possible that this is working and I'm not fetching tasks based on their list?

这是我导航到列表时的获取请求：

This is my fetch request when I navigate into a list:

if (fetchedResultsController != nil) {
    return fetchedResultsController;
}

// Create a fetch request.
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
// Create an entity so fetch the data from.
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Task" inManagedObjectContext:self.managedObjectContext];
// Set the entity of the fetch request.
[fetchRequest setEntity:entity];
// Set the amount to be fetched at a time
[fetchRequest setFetchBatchSize:20];
// Create a sort descriptor.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"task" ascending:NO];
// Attach the sort descriptor to the fetch request.
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];

// Create a fetch result controller using the fetch request
NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                                                                              managedObjectContext:self.managedObjectContext
                                                                                                sectionNameKeyPath:nil
                                                                                                         cacheName:nil];
self.fetchedResultsController = theFetchedResultsController;
theFetchedResultsController.delegate = self;

// Perform fetch.
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error]) {
    // Handle error.
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    exit(-1);  // Fail
}
return fetchedResultsController;

推荐答案

您需要跟踪拥有的列表导航到。做到这一点的一种方法是像这样创建一个名为 parentList 的属性

You need to keep track of the list that you have navigated into. One way of doing it would be to create a property called as parentList like so

@property(nonatomic,strong) List *parentList

在视图控制器中创建任务

in the view controller you create a task in. And just before navigating to the view controller set this property.

在任务视图控制器中，您执行与 List 类似的插入操作。 code>对象，使用 Task * reqdTask = [NSEntityDescription insertNewObjectForEntityForName：@ Task inManagedObjectContext：context]; 然后设置所有值一次，只要说

In the task view controller you do a insert similar to the List object using Task *reqdTask = [NSEntityDescription insertNewObjectForEntityForName:@"Task" inManagedObjectContext:context]; and then set all the values once say Save button is pressed.

[parentList addTaskObject: reqdTask];

完成。这将在任务实体中创建一个 Task 并将其映射到 List 实体。希望对您有所帮助。

and ur done. This will create a Task in the task entity and map it to the List entity. Hope this helps.

**编辑***

您需要在保存上下文之前，先执行 [parentList addTaskObject：reqdTask]; 。

You need to do this [parentList addTaskObject: reqdTask]; before saving your context.

将此内容添加到 NSFectResultsController

[fetchRequest setPredicate：[NSPredicate predicateWithFormat：@ list =％@，self.parentList]];

所以会是这样

// Create a fetch request.
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
// Create an entity so fetch the data from.
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Task" inManagedObjectContext:self.managedObjectContext];
// Set the entity of the fetch request.
[fetchRequest setEntity:entity];
// Set the amount to be fetched at a time
[fetchRequest setFetchBatchSize:20];
// Create a Predicate.
[fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"list = %@",self.parentList]];
//continue

这将带来与所选列表相关的任务。

this will bring the task associated with the selected list.

这篇关于一对多关系：CoreData的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！