我不知道我对Core Data关系的理解是否有缺陷,因为我似乎无法实现我想做的事情.
我创建了一个2个实体来管理应用上的聊天和用户和消息之间的一对多关系.所以用户可以有很多消息,但消息只是你的用户(创建者).
当添加新消息时,我尝试更新ChatUser实体关系,从而建立了ChatUser ID和ChatMessage之间的连接.我可以做到这一点,但是当我去添加一个新的消息到一个现有的userId时出现问题.目前正在实现的一切都是在ChatUser中添加一个额外的userId,而不是仅添加与现有UserId的关系.
NSManagedObjectContext *context = [self managedObjectContext]; NSError *error = nil; // 4 . Get Timestamp for Rippll float timestamp = @([[NSDate date] timeIntervalSince1970]).floatValue; NSString * jayID = @"eu-west-1:be6457ce-bac1-412d-9307-e375e52e22ff"; NSString *message = @"Science string!"; // Create a new managed object ChatUser *chatUserManagedObject = [NSEntityDescription insertNewObjectForEntityForName:@"ChatUser" inManagedObjectContext:context]; Chat *chatManagedObject = [NSEntityDescription insertNewObjectForEntityForName:@"ChatMessage" inManagedObjectContext:context]; Timeline *timelineManagedObject = [NSEntityDescription insertNewObjectForEntityForName:@"Timeline" inManagedObjectContext:context]; // 3 . Save Timeline [timelineManagedObject setEvent:chatEvent]; [timelineManagedObject setTimestamp:[NSNumber numberWithFloat:timestamp]]; [timelineManagedObject setMeta:@""]; [timelineManagedObject setViewed:@NO]; [timelineManagedObject setEventID:jayID]; //Save UserMessage [chatManagedObject setChatId:jayID]; [chatManagedObject setTimestamp:[NSNumber numberWithFloat:timestamp]]; [chatManagedObject setMessage:message]; [chatManagedObject setMedia:@""]; //Check if value exists NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"ChatUser"]; [request setPredicate:[NSPredicate predicateWithFormat:@"userId = %@",jayID]]; [request setFetchLimit:1]; NSArray *entities = [[context executeFetchRequest:request error:&error] mutableCopy]; if (entities.count == 0) { NSLog(@"GOOD TO ADD"); // no matching object [chatUserManagedObject setUserId:jayID]; //Create Relationship [chatUserManagedObject addChatObject:chatManagedObject]; } else { NSLog(@"IT EXISTS!"); [chatManagedObject setChat:chatUserManagedObject]; } // Save the object to persistent store if (![context save:&error]) { NSLog(@"Can't Save! %@ %@",error,[error localizedDescription]); }
解决方法
我瘦k,@PangHoMing在正确的轨道上,但使用了Magical Records.我们来完成CD：
首先重命名关系.可能在ChatMessage中应该有一个名为user(或chatUser)的to-1关系,而在ChatUser中应该有一个to-N关系消息(或chatMessages).他们应该是反向关系.
接下来,您应该在创建用户之前询问用户的存在. (否则你创建幻像用户.)你得到它的代码：
// Create message as you did
ChatMessage *message = …;
…
// The user will go here
ChatUser *user; // Do not use types in names unless conversion is subject of your code
// Look for an existing one
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"ChatUser"];
[request setPredicate:[NSPredicate predicateWithFormat:@"userId = %@",jayID]];
[request setFetchLimit:1];
NSArray *entities = [[context executeFetchRequest:request error:&error] mutableCopy];
if (entities.count == 0)
{
// Only if there is none,create one
user = [NSEntityDescription insertNewObjectForEntityForName:@"ChatUser" inManagedObjectContext:context];
// Set-up user's properties
…
}
else
{
// Use the existing one
user = entities[0];
}
[message setValue:user forKey:@"user"]; // message.user = user;
如其他人所提到的,CD的反向关系.但是,您可以使用反向关系来添加消息,如果这更可读：
[[user mutableSetValueForKey:@"messages"] addObject:message]; // [user addMessagesObject:message]
这也将维持“原”关系.
