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.