Add new attribute to existing Core Data Entity Relationship

Question

I'm not sure if my understanding of Core Data relationships is flawed as I can't seem to achieve what I want to do.

I have a 2 entities created to manage Chat on the app and a one-to-Many relationship between the users and the messages. So a user can have many messages but the messages have just you user (creator).

enter image description here

I am trying to update the ChatUser entity relationship when a new message is added whereby a connection between the ChatUser ID and the ChatMessage is established. I can do this but the issue arises when I go to add a new message to an existing userId. All that is currently being achieved though is adding an extra userId into ChatUser instead of adding only the relationship to the existing UserId.

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]);
}

If 'chat' is your inverse relationship from the message to the user, then all you need to do is assign the user object to the chat property of the chat message object and Core Data will take care of it for you. — Paulw11, Apr 11 '15 at 23:10
@Paulw11 Then how will core data know the value that it is related too? — memyselfandmyiphone, Apr 11 '15 at 23:12
@Paulw11 is that not what I have done with `chatUserManagedObject.userId = jayID;` ? — memyselfandmyiphone, Apr 11 '15 at 23:15
No, you don't assign the object's "id", you assign the actual NSManagedObject instance — Paulw11, Apr 11 '15 at 23:54
One other thing that seems to be confusing is that your id columns (from your server?) are not what Core Data uses to maintain the relationship connections. It has its own id and foreign key columns, hidden from us as programmers. — Eric Hedstrom, Apr 12 '15 at 07:21

jszumski · Answer 1 · 2015-04-29T14:19:19.057

6

Core Data will automatically manage the other end of the relationship once you initiate it from either direction. You can either add to ChatUser's chat collection or set ChatMessage's chat with an instance of an NSManagedObject and that's all you have to do. Note that this process does not require you to manage foreign keys, that complexity is abstracted from you. You can still store server-side IDs if they help you interact with an API, but they are not required for Core Data.

I would recommend changing one end of the relationship so that both are not named the same thing. Consider updating ChatUser's relationship to be messages or something similar.

edited Apr 29 '15 at 14:19

answered Apr 22 '15 at 19:14

jszumski

7,430
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I don't understand whats wrong with the code then? What do you mean by `ChatUser's chat collection` The data is getting added to a existing relationship but this occurs away from where the actual nsmanagedobject would get called – memyselfandmyiphone Apr 23 '15 at 10:29
You said "the issue arises when I go to add a new message to an existing userId" can you specify what the issue is? I got the impression you were just wondering how it works instead encountering a specific error? – jszumski Apr 23 '15 at 12:08

Amin Negm-Awad · Accepted Answer · 2015-04-29T14:11:19.263

I thin k, @PangHoMing was on the right track, but used Magical Records. Let's do it solely with CD:

First rename the relationships. Probably in ChatMessage there should be a to-1 relationship named user (or chatUser) and in ChatUser there should be a to-N relationship messages (or chatMessages). They should be inverse relationship.

Next you should ask for the existence of a user before creating it. (Otherwise you create phantom users.) You got the code for it:

// 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;

As mentioned by others, the inverse relationship is maintained by CD. However, you can use the inverse relationship to add the message, if this is more readable for you:

[[user mutableSetValueForKey:@"messages"] addObject:message]; // [user addMessagesObject:message]

This will maintain the "original" relationship, too.

score 1 · Answer 3 · answered Apr 12 '15 at 07:18

1

You should first check if there is an ChatUser entity already exists in your Context. Simply by trying to get the ChatUser with that userId, if there is a ChatUser return that mean that ChatUser is already been crated; if it return nil that means there isn't a ChatUser entity with that userId being created.
To handle 1 - many relation, you should use [message setChatUser:chatUser] instead of chatUser addMessageObject

Feel free to ask any follow up question if you have any. Cheers!

answered Apr 12 '15 at 07:18

Pang Ho Ming

1,299
10
29

Hi. Your code at the end doesn't make sense. As I understand it I am to set NSManaged object and those are a mix of strings and NSmanagedobject? – memyselfandmyiphone Apr 12 '15 at 11:08
I also don't have `setChatUser` – memyselfandmyiphone Apr 12 '15 at 11:09
sorry my code above are code using MagicalRecord framework. You may take a look at this tutorial, http://code.tutsplus.com/tutorials/core-data-from-scratch-relationships-and-more-fetching--cms-21505 – Pang Ho Ming Apr 12 '15 at 11:41
I have looked at that. I just don't understand how the NSManagedObject knows its connected to the existing User. – memyselfandmyiphone Apr 12 '15 at 11:55

score 1 · Answer 4 · answered Apr 29 '15 at 15:07

Try to implement something like this one. This solution is almost the same as proposed by Amin Negm-Awad

NSManagedObjectContext *context = [self managedObjectContext];
NSError *error = nil;

// 4 . Get Timestamp for Rippll

NSString * jayID = @"eu-west-1:be6457ce-bac1-412d-9307-e375e52e22ff";

NSString *message = @"Science string!";

//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];

ChatUser *chatUserManagedObject = nil;

if (entities.count)
{
    NSLog(@"IT EXISTS!");
    chatUserManagedObject = entities.firstObject;
}
else
{
    NSLog(@"GOOD TO ADD");
    chatUserManagedObject = [NSEntityDescription insertNewObjectForEntityForName:@"ChatUser" inManagedObjectContext:context];
    [chatUserManagedObject setUserId:jayID];
}

// Create a new managed object

Chat *chatManagedObject = [NSEntityDescription insertNewObjectForEntityForName:@"ChatMessage" inManagedObjectContext:context];

//Save UserMessage
[chatManagedObject setChatId:jayID];
[chatManagedObject setTimestamp:[NSDate date]];
[chatManagedObject setMessage:message];
[chatManagedObject setMedia:@""];

[chatUserManagedObject addChatObject:chatManagedObject];

// Save the object to persistent store
if (![context save:&error]) {
    NSLog(@"Can't Save! %@ %@", error, [error localizedDescription]);
}

Add new attribute to existing Core Data Entity Relationship

4 Answers4